Complex sentence: The Severn Tunnel rail accident occurred on 7 December 1991 when the 08:30 London Paddington to Cardiff Central HST was stopped at a signal guarding the entrance to the Severn Tunnel.
Split into simpler sentences: The Severn Tunnel rail accident occurred in the tunnel on 7 December 1991. The 08:30 London Paddington to Cardiff Central HST was stopped at a signal guarding the entrance to the Severn Tunnel.