Adhesion on the railway, put in simple terms, is a measure of the Traction, or slipperiness, between the wheel and rail. We can measure adhesion levels and a value of adhesion is assigned normally expressed as ‘μ ‘ (a decimal fraction) or sometimes as a percentage.
This μ value is approximately equivalent to the maximum possible rate of deceleration of a given train, when expressed as the percentage of deceleration due to gravity (g). This approximate relationship makes understanding the effects of adhesion on train braking much easier.
For example, a modern disc-braked train has a nominal braking rate of about 9%g with a Full-Service brake application. Therefore, when all axles are braking their own weight, we need an adhesion level of at least 0.09 (9%) to support this braking rate without suffering wheel slide during braking. Note that, for simplicity, in the remainder of this manual we shall refer to the adhesion level as a percentage.
For traction purposes however, the adhesion level needs to be higher to start a train without the wheels spinning, ranging from 0.15 (15%) for a typical 4 car multiple unit, up to about 0.25 (25%) for a locomotive hauling a heavy freight train. This depends on a number of factors such as the number of motored axles, the axle loads and the trailing load, etc.
In dry weather with clean (shiny) uncontaminated rails, the adhesion level would commonly be found to be between 20% and 40%, in really wet conditions it may be between 10% and 20%.
In both of these circumstances braking problems should not normally be encountered. However, adhesion levels lower than that required for Full Service braking are encountered from time to time, particularly in the autumn when moist crushed leaves on the rails can reduce levels to as low as 1%.