On 7 December 1991, two trains collided inside the Severn Tunnel, between England and Wales.
[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.
The official report into the accident[2] could not reach a firm conclusion, but speculated that the cause was either: