All modern roller coasters with more than one train operate with a system of safety "blocks" on the track. A computer will not let a train on a segment, or block, of track unless it can determine that the preceding train has left that block. A system is in place to prevent a train from entering a block. This can be accomplished by block brakes or just stopping a train on a lift hill. Smiler's computer rightly detected the stalled train that was still occupying a block of track, so it stopped a full train on the lift hill. The ride operators assumed that the computer system was in error, and manually over-rode the safety mechanism. This restarted the lift hill and sent the full train down the course. This train struck the stopped, empty train. 16 riders were injured, five were seriously injured, and two young ladies required partial leg amputations.
In the aftermath of the accident, Alton Towers was closed for four days, and the coaster was closed for about nine months.
©2017 Joel A. Rogers.