The Thameslink train had been held at a danger (red) signal until the route was set for it to cross from the Brighton Main Line onto the London Bridge Up Loop Line, and the signal then cleared. The Connex train, which was approaching from behind, passed a preliminary caution (double yellow) signal then a caution (yellow) signal and then a danger (red) signal. It continued for 283 metres (309 yards) past the red signal until the point where the two lines converged. It was travelling at an estimated speed of when it collided with the Thameslink train which was estimated to be travelling at around . Both trains were derailed and damage was caused to most of the carriages.