British Transport Police and rail company managers say the youngsters are risking their own lives and those of others.

But the rail company c2c says a special anti-vandalism train - the Q-train - has helped cut the number of attacks and cases of trespass on the Essex to London line.

The train patrols the route and carries police in an attempt to catch vandals and trespassers red handed.

Children "know they are being watched"

Crime liaison officer for c2c, Barry King, said: "We have to get the message across to children that if you get in trouble on the railway you can be taken away in a body bag.

"We do use shock tactics when we give presentations, but you have to be cruel to be kind.

"We are not here to give children nightmares. We are here in the hope that we will save lives."

Nine under-16s were killed due to trespassing on Britain's railways in 2000-2001.

'Play chicken'

But Railtrack workers say they have been threatened by older teenagers when they challenge them about trespassing.

The youngsters go on to the track to play "chicken", put objects on the line and to daub graffiti.

Mr King said within the past three months damage put at £148,000 had been caused to trains on the line due to stone throwing.

The Q-trains slot between normal passenger services and patrol identified "hot-spots" which are known to have a history of trespassing or vandalism.

The name Q-train originates from the World War I ships which were used to entice German U-boats to the surface to protect merchant vessels.