There are two types of railway track superstructure from the point of view of the response to rail temperature variation:
- free thermal expansion (FTE) track superstructure which allows the rail to freely vary its length due to temperature variation. This superstructure does not provide reliable constant and easy to control and maintain track longitudinal resistance and joint resistance. Almost all old types of fastenings and joints are of this type. These old superstructure components are fit for short rails.
- restrained thermal expansion (RTE) track superstructure – this superstructure has fastenings and joints designed to provide a well-defined resistance to rail thermal variation. Almost all modern fastenings and joints are designed in this way.
For this type of superstructure there are two main resistances that oppose the rail length thermal variation:
- joint resistance
- track longitudinal resistance
The joint longitudinal resistance will not allow the joint gap to vary until the temperature variation will cause a thermal axial stress that will overcome this resistance. After this happens, the joint gap starts to vary and the next set of superstructure elements comes into place to resist rail expansion or contraction.
The track resistance to the longitudinal and lateral rails tendency of movement, under the action of these forces, is secured through the action of the rail fastenings and the constraining effect of the boxing ballast.
The track longitudinal resistance, LR, has three levels of action:
1. between the rail and the fastening – LR1
At this level the longitudinal resistance is defined by the friction between the rail and the fastening components. For all modern fastenings the resistance at this level is well defined. On ballasted track, the longitudinal resistance of a fastening is defined by the European Norm EN 13841-2:2012 to be at least 7 kN. This is equivalent to a distributed longitudinal resistance of around 12 kN /m of rail.
The fastenings for high speed track (V>250 km/h) the minimum longitudinal resistance is 9 kN (around 15 kN /m of rail).
Special fastening systems are sometimes required on very long bridges with direct fixing, or in other specific cases. These fastenings can provide reduced longitudinal resistance (LR1), to separate the thermal expansion forces of the track from the ones of the structure. A well known such fastening system is Pandrol Zero Longitudinal Restraint (ZLR).
If on the rail is exerted a longitudinal force above this resistance and the fastening is kept fixed, the resistance is overcome and the rail starts to move relative to the fastening (EN 13146-1:2012).
2. between the fastening and the sleeper – LR2
At this level is encountered the highest resistance. For Pandrol fastenings for example, the clip shoulder is embedded in the sleeper’s concrete so no relative movement of any kind will happen at this level. The alternative, screwed fastening, is similarly good and all modern screwed fastenings are designed to have a very high lateral resistance at this level. Since the resistance at this level is significantly higher than the other two, it is usually ignored in the calculations that model the railway track thermal behaviour.
3. between sleeper and ballast – LR3
At this level the longitudinal resistance is usually the lowest and the most difficult to control. This resistance is mainly dependant of the shape, dimensions and weight of the sleeper and also on the ballast compaction and content of fines. This resistance also is not homogeneous along the track – there can be spots with high or low resistance at this level even for apparent similar track conditions.
The Track Longitudinal Resistance is the minimum of the three ( LR1 , LR2 , LR3 ) and the movement due to temperature variation will manifest at the level of that lowest resistance.
For ballasted track, there is this relation:
LR2 > LR1 > LR3
Usually the thermal expansion will hence manifest between the sleeper and the ballast. As the rail expands or contracts, it will carry with it the sleepers, moving them slightly back and forward through the ballast. If the ballast is frozen or there are other reasons that provide higher resistance at this third level, then the rail will start to move in relation to the sleeper.
- BS EN 13146-1:2012+A1:2014. Railway applications – Track – Test methods for fastening systems. Part 1: Determination of longitudinal rail restraint.
- Radu, C. (1999). Cai Ferate – Suprastructura Caii (Railway – Track Superstructure) – Course notes, Faculty of Railways, Roads and Bridges – Technical University of Civil Engineering Bucharest