Children and young people are to blame for an alarming rise in serious and fatal accidents on the railways, according to Railtrack.
Children's increasingly loutish behaviour on the way to and from school has forced operators to employ security guards and divert trains.
"The number of serious incidents on railway lines has increased and young people are causing the majority of the problems," said a spokesman for Railtrack, the privatised firm which is responsible for railway lines across the country.
A report by the Health and Safety Executive reveals a 50 per cent increase in train incidents caused by "malicious action". There were 479 in 1997, compared with 321 the previous year.
The British Transport Police say train obstructions and dangerous behaviour by schoolchildren are now a daily occurrence, and it is constantly trying to come up with new ways to counter the problem.
Shopping trolleys, piles of concrete and bicycles are placed on railway lines, and stones are thrown at carriages by pupils on overhead bridges or on the track.
In the past six weeks, two 12-year-old boys have died on train lines in separate incidents in Stretford and Tamworth, Staffordshire. Meanwhile, trains have hit oil drums, concrete posts and traffic bollards left on the tracks by schoolchildren in the past month.
A spokesman for the BTP said: "The majority of these incidents are down to children aged between six and 17. If we find we have a problem with children from a particular school, then we put police on the trains its pupils use. This stops the problem temporarily but once the police are no longer there the problems come up again."
In Wimbledon, south London, train operator Thameslink had a problem with pupils at John Major's old school, Rutlish high.
A spokeswoman for Thameslink said: "There has been an increase in vandalism by schoolchildren on trains, including throwing stones at trains, breaking into the safety equipment, and tearing out seats to build dens in unused stations.
"We've had a problem in Wimbledon with children from Rutlish high school and have had to employ security guards to man the trains during the hours when children come out of school.
"We have also reduced the amount of time that trains stand on the platform because that gave pupils the opportunity to run riot up and down the train."
Thameslink has now begun a programme of demolishing waiting rooms on unmanned stations because they were being vandalised by schoolchildren.
In the past 12 months, Connex South Central has experienced a 15 per cent rise in vandalism, assaults and graffiti on its Brighton to Chichester line.
Police started work six months ago to try to reduce the problem in the 18 towns linked by the line. They want to try to make parents take responsibility for their children.
"Gangs of kids were loitering in stations from the time they left school until 11pm," said Inspector Nick Morriss, of the BTP in Brighton.
"They were smashing windows and vandalising anything they could get their hands on."
"We started contacting the parents of children found hanging around in stations and asking them to come and do something about them. Most of the parents had no idea their children were loitering in stations - they thought they were at a friend's house. Most were concerned and came to collect their children immediately."
A spokesman for Merton Council said: "We are outraged at Thameslink's accusations. This is news to the school and to the authority. We have received no complaints whatsoever."