A roller coaster is said to have stalled, a roll back, saddle, 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.
A coaster well known for its roll backs is Top Thrill Dragster, for which this is a normal occurrence. While rolling back is normal, complete stalls are not as common on this ride. Only three times as of May 25, 2009 has the ride stalled at its apex, leaving passengers stranded until a maintenance worker has been able to push the train forward by hand.
In the summer of 2005, a steel roller coaster operating at Six Flags Magic Mountain in Valencia, California called X2 had the same problem.
Other rides have had similar stalls, and when these occur on loop coasters, the result can be traumatic, with riders suspended upside down for hours until they can be rescued.