If the slide is significant, it can heat up the steel of the wheel sufficiently to permanently modify its crystalline structure to become more brittle (martensite) which leads to cavities forming in the wheel.
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.