Rail passengers were held on a train for more than three hours when it broke down in a tunnel.
Tuesday’s 6.12pm train from London Euston to Tring stopped when an overhead power line lost power inside Watford Tunnel.
Passengers could not make calls as there was no phone signal in the tunnel.
Rail operator London Midland said the train should have restarted by itself but it was damaged.
Spokesman Francis Thomas said there would be a thorough investigation and anyone on the train is entitled to receive compensation.