Wheel slide protection

Wheel slide protection and wheel slip protection are railway terms used to describe automatic systems used to detect and prevent wheel-slide during braking or wheel-slip during acceleration. This is analogous to ABS and traction control systems used on motor vehicles. It is particularly important in slippery rail conditions.

Sanding is one method of reducing wheel slide. Locomotives and Multiple units have sandboxes which can deliver dry sand to the rails in front of the wheels. This may be initiated automatically when the Wheel Slide Protection system senses loss of adhesion, or the driver can operate it manually. Sanding may be connected to a computer system that determines the train's direction and where the sand should be applied: either forward or aft of the trucks. In older locomotives there was a manual lever attached to a valve that had three positions: Off, Forward, and Aft.

Wheel Slide Protection (WSP) equipment is generally fitted to passenger trains to manage the behaviour of wheel sets in “low adhesion” (reduced wheel/rail friction) conditions. It is used when braking, and may be considered analogous to anti-lock braking (ABS) for cars. The system can also be used to control (or provide an input to) the traction system to control wheel spin when applying power in low adhesion conditions.

“Low adhesion” at the rail potentially causes damage to wheels and the rails. Typically, low adhesion conditions are associated with environmental causes arising from seasonal leaf fall, or industrial pollution. Occasionally the cause can be another less obvious factor such as light oxidation of the railhead or even swarms of insects.

When a train is braking, the low adhesion manifests as wheel slip where the wheelset is rotating at a lower velocity (speed) than the forward speed of the train. The most extreme example of this is where the wheel stops rotating altogether (wheel slide) while the train is still moving and can result in a “wheel flat” caused by the softer wheel steel being abraded away by the harder rail steel.

However, the wheelset does not need to lock up completely in order for damage to be caused. If the slide is significant, heat can build up in the contact patch between wheel and rail sufficiently to permanently modify the crystalline structure of the wheel's steel. The steel becomes more brittle (martensite) which leads to cavities forming in the wheel. Wheel flats on railway vehicles are very evident by their characteristic “bang-bang” in time with the speed of the train. It is normally necessary to use a wheel lathe to remove a layer of wheel tread caused by a severe flat or cavity, which reduces the life of the wheel and is a major operating cost to the rail industry.

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