Passengers face delays after the "massive" power cut

Signals were knocked out for five hours between Luton and Chiltern Green at 1015 GMT on Friday.

Routes in Hertfordshire and Bedfordshire were worst-hit by the failure.

Commuters using Thameslink and Midland Mainline were warned they faced long delays into the night.

A Network Rail spokesman said: "The power is back. Everything is running but the exact cause of the problem is still being investigated."

Network Rail spent hours trying to trace the problem so they could reset the signal before trains could be allowed to run.

'Pretty major problem'

All traffic on the fast lines was stopped at 1150 GMT.

Speed restrictions were imposed on the slow lines and drivers were talked through the signals to allow some trains to creep towards their destinations.

Thameslink described the problem as a "major track circuit failure" which had forced the company to terminate trains between St Albans and Leagrave in Bedfordshire.

A Thameslink spokesman said it had been "a pretty major problem".

Half normal service

Buses ferried travellers between Luton, Luton Airport Parkway - the main station for the airport - and Harpenden stations.

By 1800 GMT Thameslink was running a skeleton train timetable to help its 21,000 rush-hour commuters complete their journeys.

Midland Mainline was operating with 30 minute delays. They were running two trains an hour from Leicester to London - half their normal service.

Passengers were advised to contact National Rail Enquiries before setting off on their journeys on 08457 484950.