Fastenings for High Speed Track

- Vipul Kumar, Executive Director/Track, Research Design & Standards Organisation, Ministry of Railways, India

1.0 Background The primary purpose of a fastener is to connect or fix the rail with the sleeper (tie). The fixing of rail to a sleeper may be done directly or indirectly with the help of fastenings. In the process the fastening is subjected to severe vertical, lateral and longitudinal forces. The forces, which are predominantly dynamic, increase rapidly with the increasing loads and speeds. In addition, vibrations are generated by moving loads mainly on account of geometrical irregularities of track and wheel and due to forces set up by the imbalance in the rolling stock. Traditional rigid fastening system of wooden and metal sleeper era were not found capable to meet the challenge of heavy dynamic forces effectively as they worked out loose under the high frequency vibrations of the order of 300 to 1000 Hertz even at a moderate speed of 100 kmph1. This type of fastening in fact is unable to hold the rail to the sleeper firmly and with a constant pressure for a good length of time. This led to development of elastic fastening system, even known initially as double elastic fastening system having resilience both below the rail in the form of a resilient rail pad, and above it in the form of a resilient clip or tension clamp, primarily acting like a spring. There had been large number of elastic fastening system developed since 1940 when the French Railway system felt the need for going for concrete ties. The elastic fastening systems had been developed initially by operating railway systems continuously improving upon them over the years based on the experience gained. Some prominent systems developed include Mills Clips and Hey Back of British Railways; R. N. Griffon and Nabla of French Railways system; K-type and Delta Clip fastener of German Railways; D.E. Clip of Netherland Railways; JRN clip of Japanese Railway; and IRN-202 of Indian Railways.

JNR Fastening system

IRN-202 Fastener (Indian Railways)

Typical K Fastener

DE Clip Fastener (Netherlands)

Nabla Fastening System of SNCF

At some stage, the process of fastening development had been primarily taken over by private players realizing the commercial potential of the product. The elastic fastening systems developed so far have more or less served well and proven their efficacy for conventional tracks of speeds upto 160 kph with axle loads upto 250 KN. On Japanese and French Railways system and also on some parts of German Railways systems, some of the fastening system developed earlier e.g. Nabla, JNR and H.M. Fastening systems have even served well for speeds upto 330 kmph for conventional ballasted tracks. This could be achieved with modifications of certain components in the fastening system e.g. rail pad for modifications in the overall resilience, (technically speaking ‘equivalent stiffness’), holding mechanism to the sleeper, reconfiguring the tension clamp of the fastening system, etc. High speed tracks normally get subjected to even greater forces and vibrations caused by higher momentum. This increases challenge for the fastening system to perform its designated functions and is the prime reason fastening system design for high speed track assuming an important role.

2.0 Pre-requisite of an Elastic Fastening System

(i) To provide an elastic link between the rail and sleeper for absorption / transfer of the rail forces to the sleeper.

(ii) Keeping the rail in rail seat position on the sleeper and provide required vertical, lateral and longitudinal stability during the disturbing dynamic forces caused on account of traffic, thermal stresses etc. In other words, it must retain the track geometry within certain tolerances against the disturbing forces.

(iii) It should have sufficient fatigue resistance for long life. Ideally, the life of each component of elastic fastening system should be equal to at least life of rail, if not the sleeper.

(iv) The fastening system should be able to maintain its properties of functional characteristics irrespective of the number of removal and re-application required for maintenance of track.

(v) The fastening system should be simple to be used by the field staff i.e. application, removal and maintenance of the fastening should be easy.

(vi) The fastening system should ideally be ‘fit and forget’ type requiring no maintenance or very little maintenance.

(vii) The fastening system should be able to provide sustainable track circuiting for the need of automatic signaling system irrespective of climatic condition.

(viii) The fastening system should have reasonable resistance against unauthorised removal (anti-vandal feature).

(ix) The fastening system should have minimal number of components from long term economic consideration to keep the inventory low for maintenance establishment.

(x) To the extent possible the fastening system should be such that it is easy to inspect and install the fastening.

(xi) The fastening system should be economical for the given traffic, climatic conditions and other socio-technological considerations pertaining to a given railway system/area. In other words, the fastening system should be such that in the long run it is economically produced indigenously with dependence on the export being minimal possible.

For high speed track the attenuation of noise and vibration to acceptable levels becomes additional requirement for environmental considerations as the frequency of secondary vibrations produced may go upto 5000 Hz2. The dynamic behavior of the track plays an important role in generating wheel-rail rolling noise. Vibrations generated are transmitted through the track and wheel structures, and these vibrations are then responsible for radiating air-borne noise. Like conventional tracks, in high speed tracks also the track geometry plays the prime role in generation of noise and vibrations with elastic fastening system having the responsibility for maintaining the track geometry. Therefore, it plays a very vital role in the whole track structure in a high speed corridor.

3.0 Design Considerations

Normal design considerations are drawn from the functions a fastenings system is supposed to perform as brought out in the preceding paragraph. Most fastening systems have evolved through empirical approach in the early stages of development with stipulations of laboratory testing of each component and fatigue test of the whole assembly for the expected loads. There have been continual efforts to draw standard criteria for design of fastening system. Symposium done on Elastic Track Fastening in the year 1992 as a special session of A.R.E.A Committee 5 (Track), Subcommittee 4 (Track Design)3 summarizes following aspects to be considered while designing an elastic fastener:

For Clip (Tension Clamp):

- Design working point of the clip (load deflection state after installation) - The elastic limit of the clip

- The spring rate (Slope of the deflection line)

The presentation underlines the following aspect while designing the clip:

The basic concept of a system design that maintains a consistent elastic (spring) resistance without permanent deformation, preferably over as large a service load range as practicable. Designers strive for an accommodating elastic range below the yield point or by deflection limiting design (building a mechanical stop).

For fastening system as a whole:

- Vertical loads which affect tie stress and rail uplift in the precession wave, therefore, laying one of the criteria for shoulder and clip and design Vertical loads also affect the pressure on the tie and in turn on the ballast thus determining the pad design in a big way. Pad design is also determined by needs of noise and vibration mitigation.

- Longitudinal loads (from traction forces and rail temperature variations) determining design of rail pad (providing required friction for mobilizing creep resistance), clip (with adequate toe load to generate required longitudinal/creep resistance) and shoulder design

- Lateral Loads which is incurred by fastening system when the L/V ratio exceeds the coefficient of friction between rail and the rail seat thus determining pad, shoulder and clip design

Design considerations for high speed fastenings system is mostly retained as protected knowledge with most of high speed railway system or their manufacturers. European National specification, however, in the final draft of EN 13481-1:2012 categories type C as a fastening system designed for conventional mail line railways with a typical axle load of 225 KN, a typical curve radius of 400 m, a typical maximum speed of 250 km/h, a typical rail section of 60E1 and a typical sleeper or support spacing of 600 mm. For higher speed (not specified by EN code, but inferred to be in excess of 250 km/h) the specification categorizes D as fastening system designed for lines with large radius curves, often used for high speed trains and having a typical axle load of 180 kN, a typical curve radius of 800m, a typical rail section of 60E1, a typical sleeper or support spacing of 600 mm and any typical maximum speed. The other corresponding EN specifications provide acceptance criteria and method of laboratory testing of various components of fastening system.

4.0 High speed fastening system for Ballastless Track (BLT)

Use of ballast less track (also known as slab track) may be inevitable in locations like tunnels, viaducts and station areas. Missing resilience of ballast in a ballastless track adds another challenge to fastening system which gets further magnified in high speed regime for reasons discussed in preceding paragraphs.

As ballasted tracks require frequent intermediate interventions for regular upkeep, many inter-city high speed routes contemplating heavy traffic are planned as ballast less track, which also provide designed control on noise vibration mitigation.

Due to ever increasing use of BLT, a large amount of research and trial work has been done and still in progress on advanced railway systems for developing ballast less track with compatible fastening system. Generally ballast track should refer to the base below the fastening system but due to the complexity involved in track response to moving loads and need for controlling combined response at design stage, fastening system design need to be considered as an integral part of whole track system. However, it is important to note that the fastening system manufactures generally are not the same ones (in the current scenario) as the firms/organizations offering ballast track design/form. Though the compatibility of each other’s system is claimed by both BLT and fastening system manufactures, track designers have to discharge huge amount of responsibility in choosing the right system for a given set of not only operating conditions in terms of speed and axle loads, but also for given alignment; civil structure chosen (tunnel, viaduct, etc.); formation and foundation conditions of nearby buildings; environmental norms to be complied; etc.

Owing to large number of variations of BLT and fastenings systems tried and used world over, a number of classifications4 are in vogue. Some of the popular classifications are brought out as under:

a. Based on Type of Civil Structure :

(i) BLT on earthwork (ii) BLT on bridges/Viaducts (iii) BLT in Tunnels

On high speed routes, design of BLT with fastening system in each of above case needs to be examined separately due to varying responses offered by these civil structures to moving loads.

Ballastless track on embankment Ballastless track in tunnel

b. Classification based on Type of intermediate base of BLT :

(i) Compact systems; BLT with concrete sleepers placed in in-situ concrete, like Rheda, Zublin, etc.

(ii) Base plated systems; BLT with elastically supported base plates on the top of the slab.

(iii) Coated concrete sleeper or concrete-block systems. These sleepers or blocks are fixed in a concrete support construction by a resilient intermediary material. This material with resilient properties can be a rubber boot or an embedding material. Sonneville or Stedef are examples of this type.

(iv) Embedded rail systems slab track with continuously-supported embedded rails in a concrete slab (with recesses) or in steel channels on bridges.

(v) Prefabricated slab systems; concrete prefabricated slabs, either reinforced and/or prestressed with any rail fastening design.

RHEDA 2000 system

Stedef Twin Block System

Embedded Rail system

Shinkansen Slab Track system

There are other classifications as well e.g. low attenuation and high attenuation BLT based on the extent of vibrations and noise attenuations; light intermediate, heavy intermediate based on the medium between two resilience; etc.

5.0 Selection of Fastening System for high speed Tracks

This is one of the most important aspects for new corridors even for conventional speeds particularly on BLT and may be very crucial for high speed tracks. There are no set rules for selection of fastening system other than checking it’s compliance to the requirements brought out in para-2 above. More organized operators do carry out ABC analysis evaluating compliance of each shortlisted fastening system through an inter-weight assigned to each parameter working out over all score and then taking final decision on cost benefit analysis. Uzbek Railways in early 2000 selected fastening system for it’s upgraded track between Tashkent and Samarkand on the basis of this method. Some operators base their decision on past proven record of fastening system which can be a very effective way of selection if the operating conditions in terms of speed, axle load, civil structure, vehicle characteristics, etc (just to name a few) are similar. However, the reliability of this method can be extremely suspicious even with minor changes in operating conditions in case of high speed corridors. Some consultants do claim that they can model track component response with varying operating conditions so as to make an ‘informed’ decision about the suitability of given track components. Clear acceptance of their claim, however is not available in public domain. Nevertheless, it is better to have some model based analysis along with proven-ness in ‘near similar’ conditions for taking final decision about the selection of fastening system for such an important track component that can make or mar the throughput capacity envisaged from the planned corridor. This is on account of the fact that the combined vehicle-track system is complex and has many natural frequencies5. When one of the excitation frequencies corresponds to a natural frequency of the system there is particularly strong vibration. As the primary vibrations are caused as a result of forces between wheels and rails, there is lot of energy associated with these primary frequencies which are normally in the range of 0-20 Hz and most civil structure may have their natural frequencies in the same range. The damage to these may be substantial if primary excitation frequencies match with some of the track components and civil structure’s natural frequencies in such a way that formation of nodes and antinodes reinforces each other rather than cancelling. This is too complex a phenomenon to be studied mathematically or even through modeling in it’s totality and adhoc decisions taken in some cases have lead to failure in operating corridors to designed capacities. Many advanced Railway systems planning high speed corridors therefore carryout detailed testing of proposed vehicle-track system through test tracks so that compatibility of vehicle, track and civil structure is substantiated before large scale commitment and investments. The Railway systems not privileged enough to have such testing tracks have to innovate and take an informed decision technically to get the right technology so that track structure can serve for next 75-100 years.

High speed corridors are normally planned between two important business cities where distance generally would exceed 400 kms needing huge amount of capital investment. In the opinion of the author, it may not be a bad idea to first construct (say) a 40-50 kms stretch of the corridor and use it as a test track to judge the efficacy of shortlisted systems, be it pertaining to track or traction or rolling stock, through extensive run over a short span of time varying from six months to a year. With so much of public money at stake it is better late by a year or so rather than run the corridor at half the designed capacity for next decade or two unless major intervention and premature expenditure solves the problem.

References:

1. ERSF-Consultancy, phase-I report to UTY by RITES Ltd., Author: Vipul Kumar

2. The Dynamic Behavior of rail fasteners by D. J. Thompson and J.W. Verheij

3. Committee 5 presentation of Elastic Fasteners, 1995 A.R.E.A Annual Technical

Conference.

4. Ballastless Track–An Overview and Developments in India by Vipul Kumar, Ashwani Kumar and Rituraj, IPWE Seminar, Chennai, January-2013

5. Modern Railway Track by Coenraad Esveld