Slip/Slide Control

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.