Track Gauge
As stated, the general track gauge in Britain was 4 ft 8 1⁄2 in (1,435 mm). In the later 1950s general track maintenance standards deteriorated rapidly due to manning difficulties, and freight train speeds increased on some routes. Freight trains consisted almost entirely of short wheelbase (10 ft) four-wheeled wagons carried on a very stiff elliptical leaf spring suspension, and these wagons showed an alarmingly rapid rate of increase of derailment events. Anyone standing at the lineside could watch a freight train pass at speed and observe several of the wagons weaving and swaying alarmingly even on good track, and derailment occurred when any poor track was encountered.
The dynamic behaviour of the wagons was the problem, but the solution adopted was to reduce the permitted speed of the wagons to 45 mph, and to reduce the track gauge by one-eighth of an inch, to 4 ft 8⅜in (1432mm) for new installations of continuously welded track on concrete sleepers. Of course the long life cycle of the track meant that this conversion process would take 30 years or more to complete. However the basis of the gauge narrowing was mistaken. The idea seems to have been to reduce the free space for lateral movement of the wagons, so that they would be "contained" to run in a straight line. In fact railway vehicles are not contained by the flanges of the wheels except in very sharp curves, and in normal running the steering effect due to the conicity of the wheels is dominant. In reducing the track gauge the effective conicity is increased – worsened – and the tendency of the wagons to yaw and roll was increased. Many derailments took place on relatively new continuously welded rail track, and often such a derailment destroyed about a mile of the new track, as the freight train might take that distance to stop; the concrete sleepers were not robust under a derailed wagon's wheels.
The effect reduced as the wagon fleet was modernised (and other effects took first place) and the track gauge for new track was quietly restored to 4 ft 8 1⁄2 in (1,435 mm) in 1998. Of course the vast majority of the track on main lines is still, as installed, at the tighter gauge, and it will be several decades before the gauge change is complete.
Read more about this topic: Permanent Way (history)
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—D.H. (David Herbert)