Severn Tunnel rail crash

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On December 7, 1991, the 08:30 Paddington to Cardiff HST was stopped at a signal guarding the entrance to the Severn Tunnel. On telephoning the signalman the driver was advised of a signal failure and given permission to proceed at caution. Three miles into the tunnel, the train was struck in the rear by a Class 155 Super-Sprinter travelling from Portsmouth to Cardiff. 185 passengers were injured, including 5 seriously.

Because track circuits were unreliable in the warm and humid environment of the tunnel, axle counters were used instead. The official report into the accident could not reach a firm conclusion, but speculated that the cause was either:

An unaccountable error on the part of the Sprinter driver, or:
technicians in the relay room at Severn Tunnel Junction had reset the axle counter while investigating the earlier fault, thus clearing the signal for the Sprinter.