On 7 December 1991, two trains collided inside the Severn Tunnel, between England and Wales. There were no fatalities but 185 passengers were injured.^[1]

The 08:30 London Paddington to Cardiff Central operated by an InterCity 125 was stopped at a signal guarding the entrance to the Severn Tunnel. On telephoning the signalman according to Rule 55, the driver was advised of a signal failure and given permission to proceed slowly at caution. Three miles (5 km) into the tunnel, the train was struck from behind by a Class 155 Sprinter travelling from Portsmouth to Cardiff. 185 passengers were injured, including five seriously, but none fatally.^[1]

Because track circuits were unreliable in the unusually wet tunnel environment (10 to 20 million imperial gallons [45 to 90 million litres] of water are pumped out per day),^[2] axle counters were used instead. The official report into the accident^[2] 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 signal failure fault, thus clearing the signal for the Sprinter.^[1]