Original Article
Rail irregularities, corrugation and
acoustic roughness: characteristics,
significance and effects of reprofiling
Proc IMechE Part F:
J Rail and Rapid Transit
226(5) 542–557
! IMechE 2012
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DOI: 10.1177/0954409712443492
pif.sagepub.com
Stuart L Grassie
Abstract
Railway noise is excited by irregularities on the running surfaces of wheels and rails, with rails being more significant for
most of the wavelength and frequency range of interest. Measurements are presented that demonstrate the significance
of irregularities in the 100–1000 mm wavelength range (corresponding roughly to 25–250 Hz) on ground-borne noise,
and show that a reprofiling specification to address reductions in low frequency noise should address these longer
wavelengths. Differences are demonstrated between irregularities in the 6.3–2000 mm wavelength range on a dedicated
high speed line, and on heavy haul, mixed traffic, metro and light rail systems. Although the sample size is small for the
high speed and heavy haul systems, the lowest levels of irregularity were nevertheless found on the high speed line. Heavy
haul systems also have low levels of irregularity, which are below the limit specified in the acoustic standard ISO 3095.
Metro systems are distinguished by corrugation, higher levels of short wavelength roughness corresponding perhaps to
roughening from consistently high traction and braking, and significant differences between high and low rails in some
curves. In one case shown here the high rail is amongst the smoothest in an entire sample of dozens of rails whereas the
corresponding low rail is by far the most heavily corrugated and irregular. Light rail systems have relatively high levels of
irregularity, particularly at short wavelengths. This may result from sanding to enhance adhesion. Reprofiling in general
reduces irregularities below the level stated in ISO 3095, except at wavelengths of less than 30 mm for which irregularities are consistently greater after reprofiling than before. Reprofiling is particularly effective in reducing irregularities in
the 30–500 mm wavelength range, in which irregularities are typically reduced by 10 dB but in some cases by 20–30 dB.
Irregularities to the ISO 3095 limit spectrum exist immediately after some forms of grinding. The best results are
obtained from reprofiling that is undertaken to an appropriate specification that is conscientiously monitored.
Keywords
Railways, railway noise, acoustic roughness, rail corrugation, roughness measurement, ground-borne noise
Date received: 14 November 2011; accepted: 6 March 2012
Introduction
Irregularities on the running surfaces of wheels and
rails initiate dynamic loads and vibration of vehicle
and track components. One of the most widely noticed
and unfortunate consequences is noise of many types
with which railways are associated and which in many
circumstances constrains development of this form of
transport, whose environmental credentials are otherwise exemplary. The literature in this area is vast with
much of the work now being basic reference material.
For example, the recent book by Thompson1 provides a
topical, practical and wide-ranging view of the area
by an acknowledged expert, with treatment of the
different types of noise that are excited by irregularities
(ground-borne, structure-borne and rolling noise) as
well as squeal and aerodynamic noise, whose mechanisms of excitation are fundamentally different.
Contributions to two recent ‘best practice’ manuals
give a useful summary not only of mechanisms of
wheel/rail noise generation but also of some of the
more effective mitigation measures for both air-borne
and ground-borne noise.2–4 This work is some of the
RailMeasurement Ltd, Cambridge, UK
Corresponding author:
Stuart L Grassie, RailMeasurement Ltd, The Mount, High Street, Toft,
Cambridge, CB23 2RL UK.
Email: [email protected]
Grassie
543
Figure 1. Simplified model of railway noise generation.
most recent since a groundswell of activity in this area
was initiated in the 1970s, giving rise to the first
Workshop on Railway Noise in Derby in 1976. The
International Workshops on Railway Noise have
continued at 3-yearly intervals since, providing a rich
and readily available source of reference material on
wheel/rail noise. Many of the references here are from
these workshops.
The significance of irregularities in generation of
structure-borne, ground-borne and rolling noise is commonly recognised. There are several published works
showing measured spectra of roughness on rails and
wheels and others where spectra have been assumed.
Some of this work is discussed below. The review is
intended to give not only an historical view of the
area but also to explore the more significant contributions to measurement of rail roughness. The vast
majority of available references are for relatively short
wavelengths (typically less than about 0.3 m). This is
barely adequate for representing the irregularities that
contribute to structure-borne and rolling noise. It is an
inadequate wavelength range for ground-borne noise,
as demonstrated in the section ‘The influence of irregularities on ground-borne noise’. A few exceptions show
irregularities to significantly longer wavelengths. There
are some references that show differences in railhead
roughness in different locations, on different types
of track or the effects of grinding on corrugation.
However, there is no comprehensive reference that
shows the range of irregularities that can exist in different circumstances.
This paper attempts to fill some of these gaps by
presenting measurements from different railway systems and in different conditions. Effects of reprofiling,
both grinding and milling, are shown, and spectra are
presented to far longer wavelengths than the common
limit of a few hundred millimetres (in the section
‘Railhead irregularities: a wide ranging survey’). An
attempt is made to explain why some of the differences
exist on different types of track and also to explore the
limitations of reprofiling on reduction of irregularities
(in the section ‘Discussion’). Although some reference
is made to the literature in the complementary area of
wheel roughness and irregularities, this paper concentrates on rail roughness and irregularities.
The objective of work such as this is clearly to reduce
railway noise. The principal area of concern here is
reduction of noise from reducing irregularities on
rails, with practical examples of what can be achieved.
Reference is made en passant to other methods of reducing railway noise.
Literature on railway noise and
measurement of rail roughness
‘Railway noise’ in the present context is considered to
be ground-borne, structure-borne and rolling noise.
These types of noise are excited to a greater or lesser
extent by irregularities on the surface of wheels and
rails. The work by Remington and his colleagues,
which was undertaken for the US Department
of Transportation in the 1970s, is widely recognised
to be both seminal and comprehensive in its treatment
of different noise sources on the railway.5–7 The model
of wheel/rail noise generation proposed in Remington
et al.6 is found in more elaborate form in the later
review by Remington8 which followed an extremely
fallow period in railway noise research in the USA.
Many of the open questions posed in Remington’s
review in 1988 were substantially answered by work
undertaken in Europe that gave rise in particular
to the TWINS model of rolling noise9 and the
comprehensive treatment in Thompson’s doctoral
research.10
The 1970s were also a productive period in the USA
in both ground-borne11 and structure-borne noise,12,13
primarily with regard to metro systems. This work
was also sponsored by the US Department of
Transportation and adopted a characteristically practical approach of combining an understanding of the
cause of problems with development of means of control. In the 1980s the Organisation for Research
and Experiments (ORE) of the International Union
of Railways undertook a wide-ranging study on
ground-borne noise and vibration, which included
modelling of these phenomena and practical methods
for control.14
A simple and slightly more general representation
of Remington and Thompson’s model that is relevant for present purposes is shown in Figure 1.
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Proc IMechE Part F: J Rail and Rapid Transit 226(5)
Table 1. Wavelength ranges (in millimetres) corresponding to frequency ranges of interest.
16
Audible ground-borne noise
Structure-borne noise19
Wheel–rail rolling noise18
Range of greatest human sensitivity15
Range of human sensitivity15
25–250 Hz
100–2000 Hz
100–5000 Hz
2–5 kHz
20 Hz–20 kHz
Table 2. Wavelength ranges considered in relevant European
and International standards.
EN ISO 3095
EN 15610
EN 13231-3
Minimum (mm)
Maximum (mm)
3.15
3.15
10
630
250
1000
This distinguishes three general areas in the mechanism
of railway noise generation:
. excitation, from wheel and rail irregularities;
. the dynamic response of the vehicle/track system,
which comprises two parts:
generation of dynamic forces as a result of
irregularities;
the response to these dynamic forces, which takes
different forms depending on the specific type of
noise of interest (ground- or structure-borne noise
or rolling noise);
. propagation of noise from the vibrating surfaces to
the receiver.
A basic question that arises with regard to irregularities is the wavelength range of interest. People are usually regarded as being sensitive to noise in the frequency
range 20 Hz – 20 kHz, with the most sensitive range
being about 2–5 kHz.15 On the other hand, the frequency
range for audible ground-borne noise is about
25–250 Hz.16 The frequency range of importance
for rolling noise is considered to be 200–2000 Hz
by Galaitsis and Bender17 or 100–5000 Hz by
Thompson.18 From the review by Wittig13 it could be
assumed that the frequency range of interest for structure-borne noise is about 250–2000 Hz, whereas the
more recent work by Bewes et al.19 indicates a slightly
broader frequency range of about 100–2000 Hz.
Frequencies of more than 2 kHz at 160 km/h are attenuated primarily by the effect of the so-called ‘contact
patch filter’, which is discussed in the ORE reports14
20 m/s (72 km/h)
50 m/s (180 km/h)
80–800
10–200
4–200
2–10
1–1000
200–2000
25–500
10–500
5–25
5–5000
and elsewhere. Frequency ranges and corresponding
wavelengths for a couple of train speeds, typical of
metros and mainline traffic, are shown in Table 1,
which illustrates the very wide range of wavelengths of
irregularity that are of interest in railway noise. The
wavelengths follow the well-known formula, ¼ =f,
where is the train speed and f is the frequency.
Three European or International standards are particularly relevant to rail irregularities and noise:
. EN ISO 3095,20 which specifies the acoustic roughness of a test section to be used for acoustic typetesting of railway vehicles;
. EN 15610,21 which prescribes a method of measuring railhead roughness;
. EN 13231-3,22 which prescribes residual irregularities that may remain on a reprofiled rail. The 2006
version of this standard is referenced as the limits of
irregularities are more relevant to noise reduction
than the generous limits in the 2011 version of the
standard.
The wavelength ranges in which irregularities
are specified in these three standards are shown in
Table 2. The reprofiling standard covers the widest
wavelength range of the three and is the only of the
three standards that is at all adequate for groundborne noise. The wavelength range of EN 15610 is
adequate for rolling noise for relatively low speed
traffic but scarcely adequate for main-line passenger
traffic.
The vast majority of available references consider
irregularities that are relevant to rolling noise or the
equivalent dynamic forces. Galaitsis and Bender17 present one of the earliest sets of measurements of both
wheel and rail roughness, with one-third octave spectra
for wavelengths of about 1.5–150 mm, which was
appropriate for rolling noise in the metro systems of
interest. Combined spectra of wheel/rail roughness,
for wavelengths of about 4.3–280 mm (125 Hz to
8 kHz at 125 km/h), are shown in Thompson et al.9
where these are used to validate the TWINS model of
wheel/rail rolling noise. Verheijen23 presents direct
and indirect measurements of rail roughness in the
Grassie
wavelength range 5–315 mm from the Dutch railway
network. The difference in the roughness spectra
between roughest and smoothest rails measured is
about three orders of magnitude over a wide wavelength range. A complementary study covering a narrower wavelength range of about 1.3–80 mm is
contained in Hiensch et al.24 Also, Nielsen25 has used
measurements of wheel and rail roughness in the
5–160 mm wavelength range to validate a model that
predicts high frequency contact forces. This work has
been extended with measurements in the wavelength
range 2–250 mm to suggest an acceptance criterion for
rail roughness levels based on both rolling noise and
development of rolling contact fatigue (RCF).26 Lower
frequency contact forces are calculated by assuming
that the longer wavelength rail roughness is that given
by the spectrum in the standard EN IS0 3095:2005.
Several sets of measurements from European railways,
shown in Fodiman and Staiger27 provided the background to limits on rail roughness that have been proposed to limit noise levels from European railway
operations. Power spectral densities of railhead irregularities for wavelengths of more than 20 mm are shown
in Grassie28 for a field investigation in which routine
measurements were made of rail corrugation development on new and ground rails.
It can be deduced from published measurements17,18
that typical wheel roughness is of a similar order of
magnitude to typical rail roughness at shorter wavelengths but of very much lower amplitude at long wavelengths. This is illustrated also in Figure 2: over the
majority of the frequency (or wavelength) range, irregularities on rails are more significant than those on
wheels. Measurements are shown for two wheels of a
passenger coach with composite brake blocks before
running and after running less than 2000 km, for the
first few hundred of which the brakes were dragging
lightly in order to remove the periodic irregularities
that resulted from reprofiling. One-third octave
spectra of roughness are also shown for a test site in
track that was measured in accordance with the
protocol in EN IS0 3095:2005. Two instruments were
used for this purpose: one based on displacement
transducers that run on a stationary straight-edge29
and the so-called corrugation analysis trolley (CAT)30
(Figure 3). Another comparison of measurements
from different instruments, including the two used
here, is included in Jones et al.31 and was made as
part of a ‘road test’ for the measuring protocol that is
now contained in EN 15610:2009. It is suggested in that
standard that instruments based on a chord of 1 m
length produce results that are acceptable only to
wavelengths of 0.100 m. Nevertheless such instruments
are often used to measure longer wavelength
irregularities.
545
Figure 2. Comparison of wheel and rail roughness.
Figure 3. The CAT in use.
The ability of an inertial instrument such as the CAT
to measure long wavelength irregularities is not similarly constrained, although there are somewhat different constraints. Record lengths can be long: almost all
samples are longer than 100 m and in one case a CAT
record of 14 km length has been used. It is nevertheless
extremely difficult to develop a traceable method of
demonstrating accuracy of long wavelength measurements because instruments such as these are amongst
the most accurate, if not the most accurate, available.
Reference is made in the open literature to other equipment e.g. a straight-edge device to measure rail and
wheel roughness,17 trolleys with displacement transducers32,33 and both a straight-edge device and an
ancestor of the CAT.28 Merits and principles of different techniques are reviewed in Grassie.34 In the last
few years the majority of references that include
546
measurement of railhead irregularities have used equipment that is commercially available and which is
described in Diehl and Holm,29 Grassie et al.30 and
elsewhere.
In view of the limitations of the straight-edge equipment that is often used to measure rail roughness, the
dearth of measurements of long wavelength irregularities relevant to low frequency noise and vibration is
unsurprising. However, a similarly successful approach
to resolving this difficulty has recently been adopted by
teams in Sweden35 and in the UK.16 Measurements of
irregularities has been made using a track recording car
(TRC) and the CAT. In both cases, the one-third
octave spectra correlate well in the wavelength range
around 1–3 m which is the upper limit of wavelength
measurable with the CAT and the lower limit with
the TRC. The measurements presented in Bongini
et al.36 demonstrate that irregularities up to at least
1 m wavelength can be measured repeatably with at
least some systems that process axlebox accelerations.
Chua et al.37 found reasonable correlation up to about
200 Hz of measured vibration levels on floating slab
track in an underground system and those calculated
assuming a spectrum of irregularities measured by
British Rail. The spectrum is rather puzzling as it contains components that are a function not only of wavelength but also of velocity. Wittig13 develops a model
that includes rail roughness to calculate structure-borne
noise, and while he demonstrates satisfactory correlation of calculated and measured noise levels he fails
to show the spectrum of roughness that has been
assumed for the calculation.
The influence of irregularities on
ground-borne noise
There is a wealth of information on the effects of
shorter wavelength roughness on rolling noise that is
sufficient to prescribe how particular sections of track
should be ground in order to maintain noise below
specified limits e.g. Asmussen et al.33 Less information
is available on the effects of irregularities on groundborne noise although there is at least colloquial acceptance that irregularities exacerbate noise and vibration.
For example, the ORE D151 study14 mentions that
wheelflats in particular increase levels of groundborne vibration while Wilson et al.11 mention rail
grinding and wheel truing amongst methods to control
ground-borne noise and vibration. Neither of these studies nor more recent work demonstrates these effects.
London Underground has hundreds of kilometres of
track that were built before noise became a problem of
public concern. On newer sections of the Underground
it has been possible to use vibration-isolating trackforms e.g. the particularly successful design at the
Proc IMechE Part F: J Rail and Rapid Transit 226(5)
Figure 4. Correlation of reductions in ground-borne noise
(dBA) and railhead roughness (dB re 1 micron), data courtesy
of London Underground.
Barbican development,14,38 which dates from the
1960s. This is not an option for older track, which
would cost billions to rebuild. About 15–20 km of
track in the most noise-sensitive locations is therefore
ground annually to reduce noise. Irregularities have
been monitored routinely by the grinding contractor
or the Underground for more than a decade.
A CAT is now used for this purpose,30 although the
Underground previously used a straight-edge device
similar to that described in Diehl and Holm.29
Measurements of irregularities before and after grinding have been obtained from four locations for which
corresponding pre- and post-grind measurements are
available of in-property noise levels. The roughness
measurements have been analysed very simply to give
root mean square (RMS) levels in short and long wavelength ranges (30–100 mm and 100–1000 mm) over
about 300 m of track adjacent to the properties of interest. For trains at a typical maximum speed of 80 km/h,
the corresponding frequency ranges are 220–740 Hz
and 22–220 Hz. The reductions in noise (in dBA) are
graphed in Figure 4 relative to the reductions in roughness (expressed as dB relative to 1 micron RMS).
Grinding brings about a reduction in ground-borne
noise of about 2–8 dB. There is a reduction in roughness of about 2–9 dB in the 100–1000 mm wavelength
range and up to about 16 dB for shorter wavelengths.
In one case there is a slight increase in short wavelength
roughness immediately after grinding.
This is a very small sample of measurements that
were taken with the independent objectives of monitoring London Underground’s grinding specification and
quantifying the benefits of grinding on noise reduction.
It is nevertheless demonstrated that there is moderately
good correlation of the reduction in ground-borne
Grassie
547
Figure 5. Irregularities on rails after reprofiling (a) one-third octave spectra of roughness and (b) relative to ISO 3095 limit.
noise with reductions in railhead irregularities in
the 100–1000 mm wavelength range, corresponding
roughly to the 25–250 Hz frequency range given in
Table 1. On the other hand, there is no correlation
between ground-borne noise and shorter wavelength
irregularities. If such a correlation were demonstrated
more widely, it would suggest that a reprofiling
specification for reduction of ground-borne noise
should not only address the wavelength range corresponding to frequencies of about 25–250 Hz but should
also ensure that reprofiling is undertaken to that
specification.
Railhead irregularities: A wide ranging
survey
It had become clear to the author from his work on
rail reprofiling and corrugation in particular that there
may be interesting differences and similarities between
548
Proc IMechE Part F: J Rail and Rapid Transit 226(5)
Figure 6. Change in irregularities on rails as a result of reprofiling.
irregularities on different types of railway track.
However, it would be impractical for anyone to measure sufficient track to discover whether trends
observed from a relatively small sample were indeed
significant. Fortunately there are many users of the
CAT instrument worldwide, and in 2010 these users
were asked if they could provide measurements from
different railway systems and in particular of track
before and after reprofiling. An excellent variety of
data was provided by many users of this equipment,
including research workers, consultants, suppliers, railway systems and reprofiling contractors. There are
measurements from many railway systems and of
rail in different condition. Measurements were provided from rail that had been reprofiled using equipment from four different suppliers of grinding trains
and one milling train. Those who contributed data are
acknowledged here, as promised, but there is no indication of which measurements come from which railway or from which type of reprofiling train. It was
clear from the measurements of reprofiled rail that a
similarly high and similarly low standard of reprofiled
rail can be supplied by every type of reprofiling equipment: it is not the reprofiling equipment itself that is
significant in providing an excellent or poor finish, but
how this equipment is used.
Measurements of the effects of reprofiling and from
different ‘types’ of railway system are presented in the
following two subsections.
Effects of reprofiling
As the CAT is widely used for quality assurance of
reprofiled rail, many sets of measurements were
obtained for this condition and in all but a few cases
the record length is significantly more than 100 m. The
basic presentation of data is of the one-third octave
spectra (Figure 5(a)). Post-reprofiling measurements
are shown for one site on a high speed line, a mixed
passenger and freight railway and a light rail system,
and for eight sites on metros. One site on a metro was
milled, all other sites were ground with conventional
modules that rotate about an axis normal to the rail.
Data are shown only if both rails had been measured.
The solid bold line in Figure 5(a) corresponds to the rail
roughness limit spectrum in EN IS0 3095:2005, which
can be expressed as follows
LISO 3095:2005
¼ 9:7 þ 18:5 log
10
¼ 9:7
for 510
for 10 4 53:15
ð1Þ
where L is the roughness limit in decibels and is the
wavelength in millimetres. This equation allows the ISO
3095 line to be extended to wavelengths beyond the
630 mm in the standard.20
Grassie
549
Figure 7. Irregularities on a dedicated high speed railway (note scale is 30 dB to þ15 dB).
Figure 8. Irregularities on heavy haul lines.
The broken bold line in Figure 5(a) is an equivalent
spectrum for the RMS limits on allowable longitudinal
irregularities after reprofiling that are specified in
EN 13231-3:2006. Although the limits are for
broad wavelength ranges of e.g. 30–100 mm and
100–300 mm, these can be recast to give the following
spectrum, which is of similar form to that in EN IS0
3095:2005.
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Proc IMechE Part F: J Rail and Rapid Transit 226(5)
Figure 9. Irregularities on mixed passenger and freight lines.
Figure 10. Irregularities on metro lines.
LEN 132313:2006
¼ 0:1695 þ 20 log
31:5
¼ 5:0515 for 255510
for 531:5
ð2Þ
The standards EN IS0 3095:2005 and EN 132313:2006 were developed with different objectives but
there is surprisingly close correlation of the two limit
spectra. The greatest difference is in the 10–30 mm
wavelength range, the reasons for this behaviour are
analysed in the ‘Discussion’ section.
An alternative and more compact presentation of the
data, which is essentially identical to that of Berggren
et al.35 is made in Figure 5(b). The limit spectrum from
Grassie
551
Figure 11. Irregularities on light rail and tram systems.
ISO 3095 (equation (1)) has been subtracted from the
data, thereby showing trends far more readily. In
particular if the post-reprofiling roughness spectrum
lies below the 0 dB line, the site has been reprofiled to
the limit specified in ISO 3095:2005. Similarly, if the
spectrum lies below the bold dashed line, the site
has been reprofiled to the standard specified in EN
13231-3:2006.
No attempt has been made in Figures 5(a) and (b) to
differentiate between measurements from reprofiled
rails at a total of 11 sites on four different types of
railway system. This has been done deliberately to
show the typical range of results that is obtained.
Although a range of finish is obtained, residual
irregularities exceed the EN 13231-3:2006 limit in only
a few cases, regardless of the type of track or type
of reprofiling equipment used. However, in almost
all cases the level of short wavelength irregularities
( < 30 mm) on reprofiled rails exceeds the ISO
3095:2005 limit.
Measurements were also available both before and
after reprofiling for the single sites on the dedicated
high speed and light rail systems, for five sites on
metros and for a single rail on a mixed traffic system.
In this case the measurements are presented as the difference in spectra of irregularities before and after
reprofiling (Figure 6). Where measurements lie above
the 0 dB line, reprofiling has brought about an increase
in irregularities; the converse is the case if measurements lie below the 0 dB line.
Irregularities on different types of railway system
The measurements presented in Figures 7 to 11 respectively are for roughness spectra on the following types of
railway systems:
. Figure 7: dedicated high speed (four measurements
from one line, pre- and post-grind);
. Figure 8: heavy haul (four measurements from both
rails on two lines);
. Figure 9: mixed passenger and freight (12 measurements from both rails on six lines);
. Figure 10: metros (eight measurements from both
rails on four lines);
. Figure 11: light rail or tram systems (eight measurements from six lines).
Differences between the roughness spectra and the
ISO 3095 limit spectrum are shown, as in Figure 5(b),
for the wavelength range 6.3–2000 mm. A few measurements are available to shorter wavelengths (Figure 9).
For the dedicated high speed line the measurements are
the same as those used in the section ‘Effects of
reprofiling’. Otherwise the measurements were not
taken directly before or after reprofiling. A ‘line’ may
be a railway system or a particular line on a railway
system e.g. the two heavy haul lines were in Australia
and the USA, whereas three of the tram ‘lines’ were
on the same tram system. For all cases except the
tram/light rail systems, spectra are shown for both
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Proc IMechE Part F: J Rail and Rapid Transit 226(5)
Figure 12. Typical irregularities on different railway systems.
left and right rails at a site. The scale on the y-axis of all
graphs is (20 dB/þ25 dB) except for Figure 7, where it
is (30 dB/þ15 dB). In Figure 12 a selection of measurements has been made for each type of railway
system, with measurements shown for both high and
low rails in a metro curve.
Discussion
Irregularities on the dedicated high speed line before
grinding are not only lower than the ISO 3095
limit throughout the wavelength range 6.3–2000 mm
(Figure 7) but also by far the lowest of the measurements shown (Figure 12). Both pre- and post-grind
measurements show evidence of an irregularity at fastener spacing at 630 mm and corrugation at about
125 mm. The irregularity at about 60 mm in pre-grind
measurements may be light corrugation. Irregularities
after grinding of this site are worse almost throughout
the wavelength range, except insofar as the light irregularity at about 60 mm is removed. Irregularities at less
than 60 mm wavelength have been particularly severely
affected by grinding.
Heavy haul lines are usually ground routinely to
reduce RCF damage. Track geometry is also maintained to a high standard to enable high loads to be
carried without damaging the track. Data are available
from only two lines, albeit from different parts of the
world on railways carrying different traffic (coal trains
and mixed freight). It nevertheless appears (Figure 8)
that routine grinding contributes to a relatively low
roughness spectrum for most of the wavelength range,
with irregularities in one case being consistently below
the ISO 3095 limit, while a combination of grinding and
maintenance of track geometry contributes to long
wavelength irregularities that are barely 5 dB above
the ISO 3095 limit: a factor of 1.8 on the RMS amplitude. For both lines there is a prominent effect of sleeper spacing at about 630 mm. Dawn39 has commented
on the fact that there is a prominent peak in groundborne vibration (which would be reflected in groundborne noise) when the sleeper-passing frequency coincides with what is now known as the ‘P2 resonance’ of
the unsprung mass on the track stiffness. This resonance is typically at 50–100 Hz, although it can be significantly lower (as in Dawn’s tests) for vehicles with a
leaf-spring suspension that is shorted out by stiction.
For one railway shown here there are prominent residual grinding marks at 25 mm, corresponding to a
grinding speed of about 5 km/h. This is relatively slow
grinding on a heavy haul railway. Spectral peaks
from grinding have been noted elsewhere.36,40 The generally low level of irregularities except at longer wavelengths is doubtless a consequence of low traffic speeds,
so that so-called ‘pinned–pinned resonance’ corrugation,41 for example, would be eliminated by contact
patch filtering.
There is a great variety of irregularities on mixed
traffic railways (Figure 9). In almost all cases irregularities are lower than the ISO 3095 limit for wavelengths
of less than about 30 mm, rising steadily with wavelength to 5–20 dB above this limit (corresponding to
an increase in mean square or RMS irregularities of
Grassie
about 3.2–100 or 1.8–10, respectively) for wavelengths
of more than a metre. This is a very wide range that
probably reflects differing maintenance practices.
In most cases the roughness for wavelengths of less
than 10 mm is similar to that on the high speed line
(Figure 7). At one site the roughness is relatively high
for wavelengths of less than 50 mm. These measurements are from a railway at high latitude where sand,
which is used for much of the year to obtain adequate
adhesion, has probably caused this high broadband
level of roughness. There is significant corrugation on
many records in the 40–100 mm wavelength range. This
probably reflects pinned–pinned resonance corrugation, with the broad wavelength range resulting from
the broad speed range for these mixed traffic railways.
In some cases there are irregularities at about 10 mm
wavelength that may be residual grinding marks from
relatively low speed grinding (about 2 km/h). Although
not directly apparent from these measurements, it can
be deduced (as pointed out by Hölzl and Werner40) that
corrugation could be initiated if grinding took place at
a speed that introduced a peak in the roughness spectrum close to the corrugation wavelength. This is a
hazard that occurs quite frequently, particularly on
metro lines, but is relatively easily avoided.
Irregularities on metro lines are generally higher
than the ISO 3095 limit at all wavelengths, with very
prominent peaks in some examples that correspond to
corrugation (Figure 10). Corrugation wavelengths are
often well defined, reflecting the tighter speed control
that is typical of metros. The worst corrugation and the
smoothest rail in this sample of measurements are actually from the same site (see also Figure 12). The level of
about 25 dB above the ISO 3095 limit in the 100 mm
one-third octave wavelength range corresponds to an
RMS corrugation level of almost 50 microns, which is a
severe corrugation indeed at such wavelengths. The
level of irregularities on the high rail is about 30 dB
lower for much of the wavelength range. Even at wavelengths of less than 10 mm, roughness is below the ISO
3095 limit only on this single sample of metro rails. One
possibility is that sustained high traction and braking
on metro systems roughen the rails. Apart from a
couple of outliers, six of the eight sets of data lie
within about 5 dB of one another for wavelengths of
more than 100 mm. This is remarkable consistency
given that the four metro lines are in different countries.
For almost all samples from tram or light rail systems, irregularities are not only significantly higher
than the ISO 3095 limit throughout the wavelength
range (Figure 11) but also greater than irregularities
on other types of railway system except for the heavily
corrugated low rail on a metro (Figure 12). There is
also a wide range of irregularities at wavelengths of
longer than about 150 mm, which is particularly
553
surprising given that three of the samples are from
the same tram system. There is evidence of corrugation
in some samples, with a quite well defined wavelength
that reflects fairly tight speed control on such systems.
Irregularities are significantly higher for almost all samples at short wavelengths, which may also reflect the
common use of sanding and its effect on roughening
rails.
In almost all the cases examined here the level
of irregularities on a rail after reprofiling are significantly below the EN 13231-3:2006 limit, as shown in
Figure 5(b). In many cases reprofiling reduces irregularities up to 15 dB below this limit in the 30–500 mm
wavelength range. In the vast majority of cases, reprofiling also reduces irregularities below the ISO 3095
limit except for wavelengths of less than about 30 mm
in which the ‘grinding signature’ and general roughness
from the grinding stones (and, in one case, milling cutters) makes it difficult to achieve an extremely smooth
surface. It is clear from Figures 5(a) and (b) that the
higher level of irregularities permitted by EN 132313:2006 in the 10–30 mm wavelength range compared
to EN ISO 3095 is not only necessary for conventional
reprofiling work but also sufficient. In two cases grinding has left a level of irregularities significantly higher
than the limit of EN 13231-3:2006 for all wavelengths
less than 100 mm. This is a poor standard of work.
Oscillating grinding is often used to reduce irregularities at short wavelengths33,42 although a technique
known as ‘offset grinding’ is also quite effective. The
effects of conventional and offset grinding are illustrated in Figure 13 from measurements made over a
400 m test site that was ground specifically to see
whether offset grinding could reduce irregularities to
the ISO 3095 limit. Measurements were made immediately after grinding and before the passage of traffic.
Irregularities were low before grinding throughout
most of the wavelength range, although greater at
short wavelengths (<30 mm) than would occur from
normal traffic e.g. Figure 12. Conventional grinding
significantly reduced irregularities in the 40–500 mm
wavelength range with a slight reduction at longer wavelength. Irregularities at short wavelength (<30 mm)
were greater after conventional grinding, with a prominent ‘grinding signature’ at 25 mm. Two passes of
offset grinding reduce irregularities over the full wavelength range 6–300 mm, with a reduction of up to 8 dB
for wavelengths of less than 30 mm, which are critical to
rolling noise. The roughness spectrum after offset
grinding exceeds the ISO 3095 limit for three adjacent
one-third octave bands (10, 12.5 and 16 mm), which is
permissible according to the standard;20 the maximum
exceedence is 2.5 dB.
The extent to which reprofiling reduces initial irregularities is informative. In almost all cases reprofiling
554
Proc IMechE Part F: J Rail and Rapid Transit 226(5)
Figure 13. Effects of conventional and ‘offset’ grinding.
Figure 14. Development of irregularities after reprofiling.
reduces irregularities in the 30–1000 mm wavelength
range by up to 30 dB, and typically by 10 dB. For wavelengths of less than 30 mm there is often an increase in
roughness of more than 10 dB. For irregularities of
more than 1 m wavelength reprofiling is also relatively
ineffective: there is a typical reduction in irregularities
of less than 5 dB (less than 50% in RMS amplitude).
The effects of milling are very similar to that of typical
grinding, with a reduction of about 10 dB in the
30–300 mm wavelength range and irregularities on the
finished rail that are within those of EN 13231-3:2006.
In one exceptional case (the high speed line of Figure 7)
grinding increased irregularities at all wavelengths on
rails that were initially extremely smooth: it is questionable why this track was ground.
It follows from the above observation that if
reprofiling increases the level of short wavelength
irregularities ( < 30 mm, say), these irregularities typically decrease under traffic. At longer wavelengths irregularities sometimes increase and sometimes decrease
depending on the type of track and traffic. This effect
has been shown in measurements presented by Hölzl
and Werner40 for a site immediately after grinding
and a year later. It is also illustrated here in a more
comprehensive set of measurements for a 100 m
length of metro line which was known to corrugate
both quickly and severely. Four sets of measurements
are shown in Figure 14, from the day following grinding through to 92 days afterwards. Although the track
was ground extremely well, there is a very small residual corrugation at 50 mm wavelength. Other prominent peaks in the one-third octave spectrum after
grinding arise from the periodic grinding marks at
16 mm, with a harmonic at 8 mm. Traffic gives rise to
a significant and continuous increase in the level of
irregularities at wavelengths of longer than about
30 mm (particularly at the 50 mm corrugation wavelength) and a very pronounced decrease in irregularities
at shorter wavelengths. The level of irregularities at
the principal wavelength of the grinding signature
(16 mm) is reduced by more than 10 dB in 3 months.
This difference in behaviour is explained, for example,
by Hempelmann and Knothe43 in their discussion of
Grassie
corrugation development. Longer wavelength irregularities or corrugation result from structural dynamics of
the vehicle/track system. Shorter wavelength irregularities, for which the corresponding frequencies do not
influence the structural dynamic behaviour, are reduced
simply as a result of wear. In essence, wear that results
from traffic reduces the level of roughness at all wavelengths unless structural dynamic behaviour exists to
increase the level. If such behaviour does exist corrugation typically develops. The spectrum of corrugation is
narrower the narrower the range of vehicle speeds over
the site.
With such a diversity of data from reprofiled track it
is difficult to determine all of the reasons for the differences. Nevertheless one finding was clear. A consistently good standard of reprofiled rail was obtained
where there existed a specification that was not only
clearly stated but also monitored using equipment
that was adequate for the purpose. This was the case
with the equipment used here, but is surprisingly
uncommon even on the majority of European mainline
railway systems. In fact the relaxation between the 2006
and 2011 versions of the reprofiling standard EN
13231-3 is in part a consequence of the fact that equipment on most European rail grinding trains is unable to
demonstrate that the requirements of EN 13231-3:2006
have been satisfied. The converse is also the case i.e.
where a standard does not exist, is irrelevant or is inadequately monitored, poor results such as the worst of
those shown in Figure 5 can result. An example that
illustrates this is that some of the worst post-grind
measurements in Figure 5 and also some of the best
come from the same metro system, before and after a
grinding specification was adopted and monitored
based on EN 13231-3:2006. An example of an irrelevant
grinding specification is one that specifies a requirement
for arithmetic mean roughness RA in the belief that this
affects noise. ‘Roughness’ of this form is reduced
quickly by plastic deformation as demonstrated, for
example, in field tests44 that provided input to EN
13231-3:2006. In these tests RA was reduced by an
order of magnitude within the running band after the
passage of less than 50,000 tonnes of traffic.
Conclusions
Irregularities on wheels and rails excite dynamic loads
and give rise to noise. For most cases and for most of
the wavelength range of interest, rails contribute significantly more to the overall roughness spectrum than
wheels. Although it is recognised that irregularities contribute to ground-borne noise, measurements that demonstrate this are extremely scarce. A contribution to
this area is made here using measurements taken by
London Underground staff of railhead irregularities
555
and in-property noise levels before and after grinding
of the adjacent underground track. It is demonstrated
that there is satisfactory correlation of reductions in
ground-borne noise levels with reductions in railhead
roughness in the 100–1000 mm wavelength range, corresponding roughly to the 25–250 Hz frequency range.
Conversely there is no correlation between reductions
in noise and short wavelength irregularities.
A wide variety of measurements is presented of
railhead irregularities in the 6.3–2000 mm wavelength
range. These come from different types of railway
system (high speed, heavy haul, mixed passenger and
freight, metro and light rail) worldwide. A comparison
is also made of the limiting spectra of railhead irregularities specified in European standards for reprofiling
of rails and acoustic type testing of vehicles. These were
extrapolated to longer wavelengths for the purposes of
this paper.
There are interesting differences in the roughness
spectra from different types of railway system. The
level of irregularities on the two heavy haul lines for
which measurements were available are compliant with
the ISO 3095 limit throughout the wavelength range
specified in the standard. Irregularities at sleeper spacing and residual ‘grinding signature’ are more prominent than on other types of line. Consistently the
lowest irregularities were found on a high speed line
and on the high rail of a metro curve. Even at wavelengths of more than a metre, irregularities on the high
speed line were about 10 dB below the limit extrapolated from the ISO 3095 limit spectrum. Conversely, the
highest level of irregularities was found, as a reflection
of corrugation, on the low rail of the same metro curve
in which the high rail had such a low level of irregularities. Mixed traffic railways have a relatively high level
of long wavelength irregularities, occasional signs of
irregularities at sleeper spacing, broad peaks in the
roughness spectra reflecting the presence of corrugation, and low levels of short wavelength roughness.
Light rail or tram systems have a high level of irregularities throughout the wavelength range, particularly at
short wavelengths. The short wavelength roughness on
tram systems, and also on one of the mixed traffic lines
examined, probably results from sanding to enhance
adhesion. There is evidence that where traffic speeds
are well controlled, corrugation occurs over a narrower
wavelength range, whereas on mixed traffic lines, for
example, the wavelengths are less well defined.
The changes in irregularities brought about by
reprofiling are shown for work undertaken by several
grinding contractors with different equipment in different parts of the world. In the majority of cases the finished rail is within the requirements of EN 132313:2006. This standard is substantially identical to the
acoustic standard ISO 3095 except that it allows a
556
higher limit of irregularities in the 10–30 mm wavelength range. The higher limit for 10–30 mm is not
only necessary in order to accommodate the irregularities that result from conventional reprofiling (both
grinding and milling) but also sufficient for the vast
majority of cases. Reprofiling in general is particularly
effective at reducing irregularities in the 30–500 mm
wavelength range, typically by 10 dB but in some
cases by 20–30 dB. It has a small influence on reducing
irregularities at more than 1 m wavelength, and brings
about a significant increase in irregularities for wavelengths of less than about 30 mm. Irregularities in this
wavelength range are subsequently reduced by wear.
Some reprofiling is done to an extremely poor standard
and in one exceptional case left the rail rougher at all
wavelengths than it was initially. Results are shown
from one technique that can achieve a roughness
within the requirements of ISO 3095 immediately
after reprofiling.
The effect of traffic following reprofiling is typically
to increase irregularities at wavelengths longer than
about 30 mm and reduce irregularities rapidly at
shorter wavelengths.
Although reprofiling usually reduces irregularities to
satisfactory limits, some of the worst results here were
obtained where there existed an inappropriate standard
that was inadequately monitored. Conversely some of
the best results were obtained where there was an
appropriate standard that addressed the irregularities
of concern and which was conscientiously monitored.
Most reprofiling work is done to a satisfactory standard, but in some cases this is more by accident than
design.
Funding
This work received no specific grant from any funding agency
in the public, commercial, or not-for-profit sectors.
Acknowledgements
It would have been impossible to produce this paper without
generous contributions from many colleagues. Those who
contributed data include ARTC, Citirail, Corus/Tata
Rail Technologies, IRT (Monash), London Underground,
Loram Maintenance of Way, Metro Medellin, Queensland
Rail, Schweerbau GmbH, SNCF, Sumitomo Metal
Technologies and Wiener Linien. The author is particularly
grateful to John Edwards and James Shepherd of London
Underground for providing the measurements of both noise
and irregularities before and after grinding, which are the
basis of the section ‘The influence of irregularities on
ground-borne noise’. The original catalyst for the paper
was an invitation from Prof Simon Iwnicki of Manchester
Metropolitan University to give the opening presentation at
a seminar on rail corrugation in April 2010.
Proc IMechE Part F: J Rail and Rapid Transit 226(5)
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