Flats

A 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.