A train which was derailed half a mile inside a tunnel is thought to have hit a slab of chalk which had fallen from the tunnel roof.

A heavily pregnant young woman was taken to hospital after the incident just outside Strood station in Kent on Thursday.

Two passengers and the driver were treated for shock by paramedics at the scene, but there were no serious injuries.

A total of 56 people were on board the 1248 GMT service from Gillingham, Kent, to Charing Cross in London, which left the tracks just outside Strood station.

It is believed the train was derailed by the debris from the tunnel roof and then collided with the side of the tunnel before the driver could bring it to a halt.

Around 35 firefighters spent more than an hour taking passengers from the train.

Passengers spoke of feeling the train hitting several bumps before hearing a big bang as it collided with the side of the tunnel.

Gina Lovering, 21, of Melbourne, Australia and her fiance Kieran Ridley, 22, of Strood, Kent, were on their way to London to travel back to Australia after visiting Kieran's family.

Praise for driver

Miss Lovering said: "There was a huge pile of chalk on the track. When we walked out of the tunnel we went past it and could clearly see it. We must have hit the chalk and then gone into the side of the tunnel."

Scott Francis, 19, of Strood, who was going to work as a printer in Wapping, east London, said: "I heard a big bang and then it took quite a long time for the train to stop."

He praised the driver and the emergency services for the way they handled the situation.

Richard Talbot, commercial manager for Connex South Eastern's Chatham area, confirmed that the debris was thought to have caused the derailment.

He said: "Railtrack have a very strict safety regime and will review everything after the full investigation."

Regular checks

A Railtrack spokeswoman said the investigation would examine whether the chalk caused the accident or fell from the tunnel roof as a result of the train being derailed.

She said the tunnel was checked every three months and was last checked at the end of October this year. It would have been due for another routine check at the end of this month.

The line itself was checked every week as it was used heavily by passenger and freight trains.

Superintendent John O'Donnell of British Transport Police operations centre in London, said a five-metre area of chalk was found on the track.

The London-bound track of Strood station was closed in order for the investigation to take place, with buses provided to take passengers from Rochester to Gravesend.

Mr Talbot said the train involved was a seven-year-old class 465 Networker, which was in a good condition.

Tens of thousands of people use the services linking Medway and North Kent with Charing Cross and London Victoria each day.