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(last edited July 20, 2005) Front Page | Recent Changes | Title Index | User Preferences | Random Page | HelpA damage spot on a wheel tread caused by the wheel locking and skidding during braking. The skidding is caused by reduced adhesion between wheel and rail and it will extend the braking distance required for a given brake application. The flats will be heard as the train restarts and will continue until the wheel treads are reprofiled in the workshop. Severe flats are considered dangerous as they may cause derailments at points so they can cause a train to be removed prematurely from service.
The problem of flats has become worse as passenger rolling stock, particularly multiple unit trains, has tended to become lighter, thus reducing the adhesive weight. Further problems have developed with the trend towards disc brakes instead of tread brakes. At least with a tread brake, the action of the block rubbing against the wheel had a scrubbing effect on the surface and helped keep it clean.
In many countries where there is a leaf fall season, the effect of crushed leaves on rails has caused significant problems with adhesion. Some lines are forced to introduce temporary speed restrictions and, in London, England, a special leaf fall timetable was used on one line where times were increased to compensate for longer braking times at stations.
