A company is investigating how a deaf and dumb teenager came to be locked in a Metro train in a dark depot.

Vicky Cheung, 18, was terrified when the driver of a late-night train failed to spot he still had a passenger on board when he ended the service.

The teenager, who can only communicate through sign language, said she was only saved because she had a mobile phone and was able to send a text message to her brother.

She had travelled with a friend on the 11.24pm service from Bank Foot station, in Newcastle, earlier this month.

The friend got off at Regent Centre, but Miss Cheung could not hear an announcement that the service terminated at that point.

Train operator Nexus was at a loss to explain how the driver failed to see a passenger was still on board.

Miss Cheung, of Sandyford, Newcastle, said she was trapped in the dark before the alarm was raised and she was rescued an hour later.

She said: ''It was the worst moment of my life. One minute the train was running along and the lights were on, and the next everything went pitch black.

''I had no idea the train was going to terminate and then go on to the depot. I didn't know where I was.

''I was sitting at the back of the train and the driver didn't see me. I knew no one else knew I was in the train and I tried shouting and banging on the door, but he didn't turn around. It was awful.''

Nexus has apologised for the incident, but has disputed Miss Cheung's version.

A spokeswoman said the teenager was only trapped on the train for seven minutes and was discovered by another driver at the depot, who managed to communicate with her using pen and paper, and offered to drive her home.