CEN/TR 15427-3:2026

• Abstract:

  This document gives the background to the requirements specified in the EN 15427 series of standards and CEN/TS 15427 series of technical specifications.

• Status: Published
• Publication Date: 20-Oct-2026
• ICS: 45.040 - Materials and components for railway engineering
• ICS: 45.080 - Rails and railway components
• Technical Committee: CEN/TC 256 - Railway applications
• Drafting Committee: CEN/TC 256/SC 2/WG 38 - Flange lubrication
• Current Stage: 6060 - Definitive text made available (DAV) - Publishing
• Start Date: 21-Oct-2026
• Due Date: 11-Nov-2025
• Completion Date: 21-Oct-2026
• Ref Project: SIST-TP CEN/TR 15427-3:2026 - Railway applications - Wheel/Rail Friction Management - Part 3: Rationale for requirements and further background information

SLOVENSKI STANDARD
01-december-2026
Železniške naprave - Trenje na stiku kolo-tirnica - 3. del: Osnovna načela za
zahteve in dodatne osnovne informacije
Railway applications - Wheel/Rail Friction Management - Part 3: Rationale for
requirements and further background information
Bahnanwendungen - Reibungsmanagement zwischen Rad und Schiene - Teil 3:
Grundlage der Anforderungen und weitere Hintergrundinformation
Applications ferroviaires - Gestion des frottements roue/rail - Partie 3: Justification des
exigences et informations générales supplémentaires
Ta slovenski standard je istoveten z: CEN/TR 15427-3:2026
ICS:
45.040 Materiali in deli za železniško Materials and components
tehniko for railway engineering
45.080 Tračnice in železniški deli Rails and railway
components
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN/TR 15427-3
TECHNICAL REPORT
RAPPORT TECHNIQUE
October 2026
TECHNISCHER REPORT
ICS 45.040; 45.080
English Version
Railway applications - Wheel/Rail Friction Management -
Part 3: Rationale for requirements and further background
information
Applications ferroviaires - Gestion des frottements Bahnanwendungen - Reibungsmanagement zwischen
roue/rail - Partie 3: Justification des exigences et Rad und Schiene - Teil 3: Grundlage der Anforderungen
informations générales supplémentaires und weitere Hintergrundinformation

This Technical Report was approved by CEN on 11 October 2026. It has been drawn up by the Technical Committee CEN/TC 256.

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United Kingdom.
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COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 15427-3:2026 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Introduction to the wheel-rail interface and the role of friction materials . 5
4.1 Wheel-rail interface and adhesion . 5
4.2 Wheel-rail interface issues . 7
4.2.1 Introduction to wheel-rail interface issues . 7
4.2.2 Wear and rolling contact fatigue . 9
4.2.3 Noise . 10
4.2.4 Rail corrugation . 12
4.2.5 Low adhesion . 15
5 Introduction to friction management and purpose of materials . 15
5.1 Purpose and function of friction management . 15
5.2 Lubrication of wheel flange or gauge face of rail . 15
5.3 Top of rail materials . 16
5.4 Adhesion materials . 16
6 Overall friction management strategy . 17
6.1 Introduction . 17
6.2 Trackside versus trainborne technologies . 18
6.3 Trackside technologies . 20
6.4 Trainborne technologies . 21
7 Flange lubricants, lubrication equipment and application . 22
7.1 Types of lubrication materials . 22
7.1.1 Flange lubricant overview . 22
7.1.2 Base oils . 22
7.1.3 Thickeners . 22
7.1.4 Additives . 22
7.1.5 Grease technologies applied to flange lubrication  . 23
7.1.6 Desirable characteristics of a flange lubricant  . 24
7.1.7 Trainborne solid stick wheel flange lubricants . 26
7.2 Trainborne application . 28
7.2.1 Trainborne Systems . 28
7.2.2 Trainborne (or on-board) systems . 28
7.2.3 Maintenance of trainborne lubricant systems . 42
7.2.4 Application strategy . 44
7.3 Trackside application . 49
7.3.1 Trackside systems . 49
7.3.2 Types of trackside systems for gauge face lubricants . 49
7.3.3 Trackside grease reservoir, pumping and control systems . 53
7.3.4 Maintenance of trackside lubricant systems . 66
7.3.5 Application strategy . 68
7.4 Testing and trials . 70
8 Top of rail materials, equipment and application . 71
8.1 Types of top of rail materials . 71
8.1.1 Introduction . 71
8.1.2 Background . 71
8.1.3 Types of top of rail material . 72
8.1.4 Surface conditioning mechanism . 74
8.1.5 Top of rail material – considerations for product selection . 74
8.1.6 Dual use products . 76
8.2 Trainborne application . 77
8.2.1 Trainborne systems . 77
8.2.2 Trainborne (or onboard) systems . 77
8.2.3 Maintenance of trainborne top of rail material systems . 86
8.2.4 Application strategy . 89
8.3 Trackside application . 94
8.3.1 Trackside systems . 94
8.3.2 Types of trackside application system – top of rail materials . 94
8.3.3 Trackside top of rail material reservoir, pumping and control systems . 97
8.3.4 Maintenance of Top of Rail Friction Management Systems . 98
8.3.5 Application strategy . 99
8.4 Testing and trials . 101
9 Adhesion materials, equipment and application . 101
9.1 Introduction . 101
9.2 Type of materials . 102
9.2.1 Dry particles (sand) . 102
9.2.2 Traction Gels . 109
9.3 Application and Systems . 109
9.3.1 Sanders . 109
9.3.2 Traction gel applicators . 125
9.3.3 Maintenance of traction gel equipment . 132
9.3.4 Traction gel application strategy . 133
10 Testing and trialling . 134
10.1 Flange lubricants and top of rail materials and application . 134
10.1.1 Introduction . 134
10.1.2 Laboratory testing . 134
10.1.3 Field testing . 145
10.2 Adhesion materials and application . 156
10.2.1 Introduction . 156
10.2.2 Sanders . 156
10.3 Traction gels . 161
10.3.1 Laboratory testing . 161
10.3.2 Field testing validation . 161
11 Glossary of terms . 162
Annex A (informative) Case studies . 166
A.1 Wear and corrugation reduction case studies . 166
A.1.1 Case study 1.1 - Use of a water based top of rail material applied from a trackside
system for reducing clip breakage, corrugations and differential wheel wear [34] . 166
A.1.2 Case study 1.2 - Use of a water based top of rail material applied from a trackside
system for reducing corrugation on a European commuter rail system[35] . 167
A.2 Case study 2.1 - Reduction in noise with the application of water based top of rail
materials and gauge face lubrication [36] . 168
A.3 Case Study 3.1 - Carrydown distance of top of rail materials . 171
A.4 Case Study 4.1 - Traction enhancing case studies . 171
Bibliography . 173
European foreword
This document (CEN/TR 15427-3:2026) has been prepared by Technical Committee CEN/TC 256
"Railway applications", the secretariat of which is held by DIN.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
1 Scope
This document provides background information, rationale, and technical guidance supporting the
15427 series of standards and technical specifications for wheel/rail friction management. It is
intended for users who are unfamiliar with friction management, those who are involved in specifying
friction management materials and/or application equipment for trainborne or trackside application,
or those who are responsible for vehicle or track maintenance. Users also include the friction
management supply industry.
Key items described in this document include:
— Fundamental principles of the wheel/rail interface, adhesion, friction behaviour, and common
issues such as noise, wear, corrugation, and low adhesion.
— The purpose, function, and performance of friction management materials, including flange
lubricants, top of rail materials, and adhesion enhancers.
— An overview of trackside and trainborne application technologies, including equipment
options, selection criteria, and operational factors.
— General guidance on maintenance, operational strategy, environmental considerations, and safe
implementation of friction management systems.
Testing and trialling methods, and performance considerations for evaluating friction
management solutions.
Overall, the document acts as a technical reference for understanding the rationale behind
requirements, correct application of friction management solutions, and their role in achieving safe,
reliable, and efficient railway operations.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
• ISO Online browsing platform: available at http://www.iso.org/obp
• IEC Electropedia: available at http://www.electropedia.org/
4 Introduction to the wheel-rail interface and the role of friction materials
4.1 Wheel-rail interface and adhesion
The wheel-rail interface is the area where all the forces are transferred between the wheel of a vehicle
and the rail. It is responsible for withstanding vertical load and transferring the dynamic forces caused
by curving, traction and braking performance of the vehicle to the track.
The contact patch between wheel and rail is relatively small in area, approximately 250 mm (Figure
1). This results in extremely high stresses at this steel-steel interface that can frequently exceed 1GPa.
In addition, this small contact patch transfers the required tangential forces between the wheels and
the rails for guiding the vehicle and for accelerating and decelerating as defined in the technical
specifications, to ensure not only the expected performance, but also the required safety margins.
Figure 1 — Demonstration of the size of the contact patch between the wheel and rail
The capability of creating these tangential forces between the materials in contact is known as friction,
which is the resistance to relative motion between two bodies in contact. Based on Coulomb's Law of
Friction, Formula (1), the higher the coefficient of friction (µ), the bigger the tangential force (F) that
the contact will be able to transfer for the same normal load (N).
F= μN  (1)
At the same time, if the required tangential forces in the wheel-rail interface exceed the available
adhesion, where adhesion is defined as the friction that can be made available to deliver tangential
forces between a driving railway wheel and the rail, this will cause a creepage between the wheel and
the rail. Creepage being the relative difference between the speeds of both components. The bigger the
exceeded tangential force, the bigger the creepage. This relation between tangential force and creepage
at each wheel-rail interface is known as the adhesion curve. As it is graphically represented in Figure
2, adhesion or the equivalent tangential force increases up to a limit value defined by the coefficient of
friction and the normal load. Once this limit is reached, it can be considered that the full contact patch
is slipping, while for smaller adhesion or tangential force values, part of this contact patch remains
rolling without any creepage.
Key
A Direction of travel
B Stick
C Slip
D Creep region
E Pure slip
F Frictional force
Figure 2 — A creep curve showing the relationship between adhesion and creep.[1][2]
4.2 Wheel-rail interface issues
4.2.1 Introduction to wheel-rail interface issues
Adhesion is essential for being able to operate railway vehicles properly and safely. Insufficient friction
prevents the desired tangential forces from being achieved, which reduces braking and traction
capabilities. For braking, this is a safety issue as it leads to extended stopping distances. In addition,
high friction is responsible for undesirable phenomena in the wheel-rail interface. A high coefficient of
friction can lead to high tangential and lateral stresses on the wheel and the rail surface, which in the
case of being repetitive, can cause fatigue in the material and initiate cracks, known as rolling contact
fatigue (RCF). Based on [3], high coefficients of friction can cause self-excited vibrations, which is the
probable cause of rail corrugation and squeal in small radius curves. However, when the coefficient of
friction is not sufficient to withstand the required tangential forces and ensure pure rolling, creepages
or even pure slipping occurs between the wheel and the rail, which together with the high coefficient
of friction can contribute to the following undesired phenomena:
— Wear
— Flange-hissing noise
— Squealing noise
— Rail corrugation [3]
— Extended stopping distances
On the one hand, traction and braking efforts are mainly transmitted by the wheel tread and top of rail
contact, so a sufficient coefficient of friction is required in this contact patch. The necessary minimum
coefficient of friction in each wheel rail contact will depend on the required traction and braking
performance and the vehicle and track features like the axle load, motorization ratio and the track slope.
On the other hand, as most critical wear rates and part of undesirable wheel-rail rubbing noise happens
as a consequence of the contact between the wheel flange and the rail gauge face, in that kind of contact,
the aim is to reduce the coefficient of friction.
Table 1 gives typical adhesion conditions that can be found on the rail. The adhesion is stated as a
coefficient of friction range and is accompanied by a description of the conditions that can lead to this
condition. It is important to note that the values stated are typical and are not to be taken as definitive.
Table 1 — Typical adhesion conditions[4]
Typical adhesion level Description of conditions
> 0.20 Clean, dry rail
0.10 – 0.20 Steady moderate or heavy rain; “wet rail”
0.08 – 0.10 Steady light or light rain; “wet rail”
Fine rain or drizzle, dew, mist, fog, snow – clean rail, light
0.06 – 0.08
leaf film contamination
Onset of rain, very fine rain or drizzle, dew, mist, fog or
0.04 – 0.06
snow, light leaf film contamination
Wet tunnel. Excessive lubricant, mist, fog, dew, drizzle,
0.02 – 0.04 onset of light rain. Snow, ice. Damp, leaf fall
contaminated in leaf fall vulnerable areas
0.01 – 0.02 Damp, leaf fall contaminated in leaf fall vulnerable areas
The purposes of the materials added to the wheel-rail interface are mainly:
— To modify the value of the coefficient of friction between the wheel and the rail to the desired levels.
— To change the shape of the adhesion curve (Figure 2).
By providing adequate materials with the appropriate strategy (time and location), the coefficient of
friction in the contact patch is enough to ensure the required traction and braking efforts in the top of
rail contact when required, whilst damage (mainly wear, RCF and corrugation) and noise are minimized
mainly in small radius curves, by reducing the coefficient of friction or changing the shape of the adhesion
curve between the wheel and rail.
Figure 3 illustrates how changing the coefficient of friction between the wheel and rail can impact on
traction, braking, noise and damage. It demonstrates that there are opposing target values which can
result in sub-optimal performance and safety issues. There is a narrow band for the target coefficient
of friction, which balances the need for reducing damage and noise and the need to maintain traction
and braking. This balance and optimal condition can be managed through the application of friction
materials.
So, as a summary, depending upon the specific contact point (rail gauge face / wheel flange or top of
rail / wheel tread) and the required operational conditions at each moment and track location, the target
could be different. i.e.:
— Flange contact: A coefficient of friction as low as possible is desired in order to reduce as much
as possible wear and plastic deformation of the wheel and rail. Additionally, the same lubricant
can be able to isolate the metallic contact and avoid the rubbing or hissing noise caused by the high
creepages between wheel flange and rail gauge face. Adding lubricant aims to minimize the
coefficient of friction.
— Tread contact when curving: On the one hand, positive or neutral gradient adhesion curves, see
section Clause 5, are required for avoiding stick-slip phenomena. On the other hand, a
recommended friction value of around 0.2-0.4 will ensure steering forces, traction and braking
efforts and it also avoids self-excited vibration of the vehicle-track system, which is responsible
for some kind of rail corrugation [3]. The application of top of rail materials helps these conditions
to be achieved.
— Tread contact under traction/braking: High coefficient of friction is desired for ensuring
required traction and braking efforts. Application of sand or traction gel in the contact patch helps
to increase a base low coefficient of friction.
The act of introducing external materials to the wheel-rail interface, considered as part of the naturally
rd
existing 3 body layer in the wheel-rail interface, could also lead to the contamination of the
environment, which could be considered as another non-desired consequence.
Some of the most relevant non-desired phenomena related to the wheel-rail interface and adhesion will
be briefly explained below, together with the role that friction materials play in mitigating them.
Key
TR Traction - desired friction for traction
BR Brake application - desired friction for braking
N&D Noise and damage - desired friction for minimising noise and damage to the rail
A Top of rail desired friction
B Gauge face desired friction
CoF Coefficient of Friction
Green Optimal friction conditions
Orange Compromised friction conditions
Red Critical friction conditions
Figure 3 — Optimising wheel-rail friction for all conditions
4.2.2 Wear and rolling contact fatigue
The wheel-rail interaction, as any other mechanical interaction between two metallic surfaces, causes
damage to both surfaces. Focusing on the most common and frequent damage mechanisms occurring
due to the wheel-rail interaction in curves, damage can be separated into two primary mechanisms:
wear and rolling contact fatigue (RCF)[5].
Wear is the loss or displacement of material from a contact surface, while the rolling contact fatigue
presents as repeated cracks on or near the surface of the wheel or rail. The wheel-rail contact patch is
divided into regions of stick and slip, as shown in Figure 2. Simplifying the wheel-rail contact mechanics
for understanding, in the regions of slip the common mechanisms of damage usually is wear, while
within the stick region the main mechanism of damage could be considered fatigue driven. In other
words, the higher the slip between wheel and rail, the more predominant will be the wear mechanism,
while in the cases in where pure rolling predominates, the principal damage mechanism will be the RCF
[5].
However, it is not only about pure rolling or slipping. Exceeding certain threshold values of forces and
creepages cause damage to the materials involved in the interaction. This threshold is dependent on
the material properties of the wheel and/or the rail. Lubrication protects the steel surfaces by reducing
the coefficient of friction between the wheels and the rail, so promotes sliding. But it also means that
tangential forces are minimized, usually pushing them below such minimum damage threshold. That is
why lubricants are applied in the wheel flange and rail gauge face contact, where adhesion is already
saturated.
Regarding RCF, the application of any lubricant or top of rail material, can also help to mitigate it as the
coefficient of friction is reduced and thus, the cyclic tangential forces responsible for causing fatigue
cracks are limited. However, as some theories show that hydraulic action exacerbates crack growth, as
any other liquids, lubricants or top of rail materials could promote crack propagation in the case that
the contact forces are not reduced enough for avoiding crack initiation.
A change in coefficient of friction in a wheel-rail interface by applying lubrication, top of rail materials
or adhesion materials in curves can change the vehicle dynamics and needs to be considered, but this
is not covered within the scope of this report.
4.2.3 Noise
Noise is a common consequence in any metal-metal contact. The wheel-rail interface is not an exception,
for example in small radius curves, where the creep gains importance and causes noise due to the flange
rubbing of the outer wheel against the rail gauge face.
In addition, the wheel-rail interface in small radius curves can cause another kind of noise, known as
“curve squeal”. The particularity of this other kind of noise is caused by exciting a natural frequency of
one or more components of the system, due to frictional instability or other external force. This noise
is not caused directly by the rubbing contact between the wheel and rail.
Curve squeal is an intense tonal high-frequency noise, which makes it undesirable to human hearing. It
is generally recognised that the majority of this undesirable noise comes from the wheel, commonly on
the inner rail of the curve.
The paper [6] written by David J. Thompson et al. in 2018 gives a detailed explanation of the curve
squeal phenomenon, but for this technical report it could be explained as follows. This stick-slip theory
is one of several theories to the source of curve squeal noise, other theories include Mode Couple
Instability Mechanisms [7][8] .
Due to the angle of attack generated in the curving of a bogie in a small radius curve, creepage is caused
both in the longitudinal and lateral direction of the wheel as it is represented in Figure 4.
Key
D Direction of travel
a Angle of attack
b Rolling velocity
c Lateral velocity
Figure 4 — Typical curving behaviour for small radius curve (a) showing that the leading
wheelset has a large angle of attack; generation of lateral creepage by a non-zero angle of attack
on the inner rail (b) [6]
At the large values of lateral creepage caused in a small radius curve, creep forces saturate the available
friction. This means that the maximum available coefficient of friction is required by the contact forces
and that adhesion reaches the negative slope section of the curve, represented in Figure 5 a). Under
negative friction characteristics, seen in a dry wheel-rail interface, the forces between the rail and wheel
build up to a level at which the wheel will suddenly slip against rail, the force will drop and the wheel
will gain traction again until the force reaches the limit and slips Figure 5 b) . Thus, the negative slope
of the adhesion curve, understood as a negative damper from dynamics point of view, is responsible for
causing the stick-slip mechanism in the wheel-rail interface, which in some cases, due to its unstable
characteristics, triggers one of the resonance modes of the wheel or the wheelset, which usually requires
an excitation in the lateral direction of the wheel, exacerbated by the angle of attack. The coupling of
the unstable stick-slip behaviour of the wheel-rail interface with a natural frequency of the wheel is the
source of the curve squeal noise. That is why this kind of noise has the particularity of being a tonal noise.
Key
a Free rolling
b Sloping portion
c Saturation
d1 Positive friction
d2 Neutral friction
d3 Negative friction
C Creep rate
F Creep force
a) Schematic diagram of the friction coefficient as a function of the slip, showing the different
elements of the curve on a traction force (F) vs. creep (C) plot
Figure 5 (continues)
Key
a Force between wheel and rail increases with increasing creep
b Past the saturation point force decrease with increased creep until wheel starts to slip
c As the wheel slips the force drops and the wheel gains traction again
C Creep rate
F Creep force
b) Diagram showing the slip-stick cycle on a traction force (F) vs creep (C) plot where negative
friction properties are present
Figure 5
Regarding the role of friction management materials, it is taken into consideration that the materials
used for mitigating the flange hissing or rubbing noise and also the wear caused by it, look to reduce as
much as possible the coefficient of friction between the outer wheel flange and the gauge face of the rail.
Nevertheless, the aim of the materials added to the top of rail and wheel tread contact in the inner rail
of a small radius curve are different.
First of all, it is necessary to guarantee a minimum coefficient of friction between the top of rail and the
wheel tread in order to be able to transmit the required traction and braking efforts between the vehicle
and the track. This allows vehicle accelerating and braking as expected. Based on this requirement, the
use of lubricant on the wheel-rail interface of top of inner rail in small radius curves are to be avoided.
Furthermore, the material added to the wheel-rail interface to the top of rail of the inner rail of small
radius curves is not cause a negative slope in the adhesion curve, in order to avoid the unstable stick-
slip phenomena and coupling it with any of the natural modes of the wheels or wheelset. Materials that
do not attempt to minimize the coefficient of friction as much as possible but have the aim of modifying
the adhesion curve, avoiding any negative slope on it, are known as top of rail materials 5.3.
4.2.4 Rail corrugation
Rail corrugation is a kind of rail damage which combines a material damage mechanism with a dynamic
response of the system. The damage mechanism can be related to plastic deformation or wear.
The causes, damage mechanisms and wavelength-fixing mechanisms of the corrugation are very
extensive, as it can be seen in Table 2 or in Ref. [3]. Corrugation can appear even in straight track or in
small radius curves. Regarding the excitation mechanisms, in some cases corrugation is strongly related
to the coupling of the vehicle-track system, which can be triggered by the stick-slip phenomenon
described previously in this document, or it can also be self-excited if a certain value of the coefficient
of friction is exceeded and the minimum exciting forces are generated in the wheel-rail interaction.
Finally, an artificial trigger can also excite a natural frequency of the system, which depending on the
receptance of it, will be damped or not.
While in some railway systems the stick-slip behaviour of the adhesion at the wheel-rail interface
couples with a high frequency natural modes of the wheel or wheelset causing curve squeal noise
(1-5kHz), the same stick-slip phenomenon on the wheel-rail interface can excite lower frequency modes
(50-500Hz approx.) of the rolling stock or track, which can have as a consequence an irregular wear of
the rails, known as corrugation.
Table 2 — Types of corrugation, characteristics, and treatments table from Grassie S.L. [2009] [9]
Type Wavelength Fixing Where? Typical Frequency Damage Mechanism Treatment
Mechanism [Hz]
Demonstrably Probably be successful
Successful
Increased pinned-
Pinned-pinned
Pinned-pinned Straight track, outer rail Hard Rail, friction pinned frequency to
resonance (roaring 400 – 1200 Wear
resonance of curves management increase corrugation
rails)
wavelength to <20mm
Friction management,
Second torsional Reduce applied traction
hard rails, reduce cant
Rutting resonance of driven Inside rail of curves 250 – 400 Wear in curving, improve
excess, asymmetric
axles curving behaviour
profiling in curves
Straight track or outer Hard Rails, high
P2 resonance P2 resonance 50 – 100 Wear Reduce unsprung mass
rail in curves resilience track forms
Reduce cant excess
Heavy Haul P2 resonance Straight track or curves 50 – 100 Plastic flow Hard Rails when corrugation on
inner rails
Light Rail P2 resonance Straight track or curves 50 – 100 Plastic Bending Increase rail strength Reduce unsprung mass
Avoid ‘peaky’
Hard Rails, Friction
Trackform Specific Trackform Specific Straight track or curves Wear resonances, improve
Management
steering
As in the case of the curve squeal noise, the application of top of rail materials to the wheel-rail interface
on the top of the inner rail in a small radius curve could help avoid the stick-slip mechanism and thus,
the excitation and/or coupling of any of the natural frequencies of the vehicle-track system, without
compromising the available adhesion.
The same effect could be achieved by using lubricants, but in that case a very low coefficient of friction
would be achieved and the capability of transmitting traction or braking efforts would be compromised,
jeopardizing the traction and braking performance of the vehicles.
4.2.5 Low adhesion
Lack of adhesion for expected traction and/or braking performance is another non-desired
phenomenon in the wheel-rail interface as it compromises the performance of the vehicle in terms of
acceleration and braking capabilities.
Basically, if the available adhesion in the wheel-rail interface is below the adhesion required by the
vehicle for achieving certain values of acceleration or deceleration, the vehicle will not be able to achieve
them, which could jeopardize in the worst case the safety of the railway system regarding braking
distance.
The wheel-rail interface is considered to be able to create a coefficient of friction of up to 0.6-0.7 in dry
conditions. Nonetheless, the introduction of a 3rd body layer to the wheel-rail interface can make this
coefficient of friction decrease to values close to 0.01, depending on its characteristics. According to
current knowledge, the most common contaminants in terms of reducing the coefficient of friction that
occur regularly are moisture, wet crushed leaf layers and/or grease.
The low friction layer created by leaf material is difficult to prevent in some environments, especially
in the autumn period, and that is why sanders and traction gel applicators are installed. They increase
adhesion up to appropriate values at which the rolling stock will be able to perform safely and without
causing wheel flats. However, sand application can be detrimental to wheel and rail wear.
The presence of uncontrolled amounts of lubricant in the top of rail and wheel tread contact can cause
low adhesion and compromise the traction and braking capabilities of the rolling stock. Any wheel
flange or rail gauge face lubrication application strategy avoids the migration of lubricant to the top of
rail and any contamination of the wheel-rail interface far from wheel flange and rail gauge face.
5 Introduction to friction management and purpose of materials
5.1 Purpose and function of friction management
This section describes three main types of friction management used to enhance wheel-rail interface
performance:
— Lubricants for the wheel flange and the gauge face of the rail
— Top of rail materials
— Adhesion materials
5.2 Lubrication of wheel flange or gauge face of rail
Lubricant is applied to the wheel flange and flange root, or to the gauge face and gauge corner of the
rail to reduce the friction between the wheel and the rail. Contact between the wheel flange and gauge
face/corner of the rail often occurs on small radius curves (typically less than 400m radius, depending
on vehicle dynamics) and switches. When this contact occurs, there are large creep (slip) rates in the
contact patch, as the radius of the flange is larger than the tread of the wheel (the contact patch of the
flange has to travel further per wheel revolution). Combined with the lateral force imparted on the rail
this sliding between the wheel and rail can result in wear, RCF damage, and/or flanging noise.
To reduce wear and noise issues caused by wheel flange and rail gauge face contact, lubricant is used.
The lubricant significantly reduces the coefficient of friction in the contact patch, reducing the tangential
force imparted on the contact. It is noted that wheel flange lubrication is not effective at reducing curve
squeal, see 4.2.3. Additionally, as with any fluid applied to the rail, lubricant could lead to an increase
in RCF crack growth rate due to the effect of hydraulic entrapment.
Although a low coefficient of friction can reduce the risk of derailment due to wheel climbing over the
rail, this is not the aim of the lubricant. When vehicle-track dynamics mean the risk of flange climbing
is high (high lateral forces compared to vertical forces), a lower coefficient of friction increases the
likelihood of the wheel to fall down the gauge face of the rail, rather than climbing over. Lubrication
cannot be used to increase any safety factors regarding the risk of flange climb derailment (such as a
change in maintenance tolerances), however it can be considered as an additional preventative measure
against flange climb derailment.
Rail lubricant can be applied from trackside systems, directly onto the gauge corner of the rail, or from
the trainborne systems, by applying it on to the wheels in order to be transferred on to the rails by
flange contact.
Lubrication summary:
Purpose: Reduce friction as much as possible in the contact patch.
Potential benefits: Reduction in rail and wheel wear, wheel and ra
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