A critical feature of the railway environment is the interface between wheel and rail. This interface is dependent upon the adhesion between the steel surface of the wheel tread and the steel surface of the rail head. The relationship is defined as the coefficient of friction. On a dry day is this is about 0.3, on a wet day 0.2 with clean rails. A figure of 0.1 is allowed for normal braking and 50% of that added as a safety margin to prevent overrunning. Values under 0.05 will occur in conditions where the rail head is contaminated by leaves or ice.
The coefficient of friction figures relate to circumstances where there is no sliding action between wheel and rail. Tests have shown that braking distances will increase considerably if the wheels slide during braking. There is nothing worse for a driver who applies the brake and then sees the speedometer drop instantly to zero. He knows he will not be able to stop in the right place. There will also be wheel damage, called flats.
Detection: In order to reduce the likelihood of excessive braking, many locomotives and multiple units are fitted with wheel slide control systems. The most common of these operates rather like ABS (automatic braking systems) on road vehicles. The railway systems usually monitor the rotation of each axle and compare rotational speeds between pairs of axles. If a difference appears between a pair of axles during braking, the brake is released on those axles until the speeds equalise, when the brake is re-applied. All this occurs automatically. Modern systems also detect too rapid deceleration of an axle. Another form of slip/slide detection uses Doppler radar techniques. This measures the ground speed of the locomotive against the revolutions of each wheelset and uses the detection of a difference to regulate the control systems.
Practice: Although the theory of the use of a wheel slide protection system during braking says that it should allow the an EMU train to stop safely within the normal braking distance for a given degree of application, in practice the effect of releasing the brakes and re-applying them many times lengthens the actual braking distance. This is because drivers usually start braking according to position and not according to speed.
A section of line over which a driver passes often will allow him to determine over time that the best point to commence braking in order to stop at, say, a station is in a particular position, using a landmark, tree, signal post bridge or something similar. This assumes that the train speed is usually the same each time he passes this point.
The theoretical concept for slip/slide control would only be valid if the driver knew in advance that the wheel rail adhesion would be reduced and made a normal application of the brakes in advance of the usual braking commencement point. Because of the reduced adhesion, the normal application would induce the wheel slide control and, instead of stopping short of the correct position as he would have done with a dry rail, the train will stop in the correct position under the control of the wheel slide protection system.
ATO: Wheel slide control has further limitations when in use on an automatic train operation (ATO) system. On suburban commuter lines, subways and metros, many of which use ATO systems, rapid braking is necessary to reduce the headway and the train control system is designed to do just this. It would require some pretty sophisticated detection systems to alert the
ATO to poor adhesion if wheel slide was to be automatically controlled and safe braking distances adhered to. An accident on the Washington subway system a few years ago was caused by a train sliding on iced rails whilst braking into a station, failing to stop in the correct distance and hitting a parked train beyond the station. Most ATO systems used on open lines have additional margins built into the braking control to compensate for poor adhesion.
In many instances, the wheel slide control is combined with wheel slip control. Wheel slip occurs during acceleration and is therefore not part of the braking system. It has however, become pretty sophisticated with creep control allowing good acceleration with virtually no equipment damage.