Stall (roller coaster)

A roller coaster is said to have stalled, a roll back, or valleyed if it is unable to complete the course because of energy loss. This can be caused by friction between the track and wheels, or in the wheel bearings. Weather can also cause a coaster to valley. Roller coaster trains run slower in cold weather, so much so that many parks actually heat the trains prior to opening on cool days. When a coaster stalls, it stops and reverses direction while approaching a high point in the track, then moves back and forth in an oscillating motion until it comes to rest at a low point of the track. Depending on the type and layout of the coaster, a stalled train may be able to be pushed back to the station, or it may have to be removed from the track and replaced in the station car by car.

The most unusual coaster stall was when Top Thrill Dragster was launched with just enough energy to come to rest at the top of the tower. Because it had stalled at a high point of the track rather than a low point, a maintenance worker was able to push the train forwards by hand.