A spot on a wheel, caused by skidding or wheel slide, which causes a tapping sound as the wheel rotates and which can usually only be removed by expensive reprofiling of the wheel on a lathe or wheel turning machine. Drivers are cautioned against too heavy braking in bad weather thus causing flats. Many attempts have been made over the years to reduce flats by wheel slide control systems which automatically reduce a brake application in the event of a slide. Such systems generally enctheage a driver to apply emergency braking which eliminates the wheel slide control and puts flats on the whole train instead of a few wheelsets. An effective solution to this problem is still awaited.