"The flat spot occurs when a rail vehicle’s wheelset is dragged along the rail after the wheel/axle has stopped rotating. Flat spots are usually caused by use of the emergency brake, or slip and slide conditions that cause wheels to lock up while the train is still moving. Flat spots are more common in the autumn and winter when the rails are slippery, but can also be caused by faulty brakes or wheelset bearings.
Once these flats are created, they provide the characteristic ‘thump-thump-thump’ that can be heard by rail passengers during autumn as the damaged wheels impact the hard rails beneath them. The sound is generated as the edges of the wheel flat impact on the rail, but, as the sharp corners of the flat are worn away over time, the noise reduces, making human detection difficult.
If the flat spot is very small, the rail vehicle will be able to continue being used. The fault is removed later in the wheelset turning process. However, because of the heat suffered while being dragged along the rail and the impacts suffered afterward, these wheels are more likely to break due to changes in the alloy structure. If the flat spot is very large, strands of molten metal may have got stuck on one side of the flat spot, making it impossible for the wheel to turn due to insufficient clearance between the rolling surface and the brake block. In this case, the wheelset must be replaced immediately. In extreme cases, a wheel with an untreated flat spot can damage the track and cause a derailment.
Otherwise smooth but ‘out of round’ wheels generate ever more force as the train gets faster until the forces become very damaging to the rails. At their extreme, these forces can be large enough to break rails which already have small cracks or defects in them."
