Children have been caught vandalising rails

The Network Rail campaign, No Messin, aims to reduce the number of young people who trespass on the tracks.

There are an estimated 27 million cases of trespass on Britain's railways every year.

Although Cardiff is one of the five worst areas for rail crime, recent incidents have been reported in Merthyr, Caerphilly, Newport and Rhyl.

Incidents across the UK include 640,000 reports of objects being placed on the tracks and four million objects being thrown at trains, some smashing windows and injuring drivers.

Network Rail's campaign aims to highlight the dangers of playing on or near railway tracks.

Train driver Scott Kenny works on the Cardiff to Portsmouth line and has been a victim of vandals.

Whilst driving his train to Maesteg, a youngster on top of a footbridge tied a house brick to a rope and lowered it so that it smashed the cab window.

"It's quite frightening travelling at 75 mph and, all of a sudden, you've got a plate glass window coming towards you," he said.

"You just put the brakes on and close your eyes and try to protect your face as best you can."

Other incidents he has been involved in include cases of cab and passenger windows being shattered by children shooting air rifles.

"Initially you are more scared and nervous than anything - worried that your passengers are safe," he said.

"It is only afterwards, with hindsight, that you get angry with the children. It does affect drivers quite profoundly."

Pc Derek Henderson, an officer with British Transport Police in south Wales, has seen many incidents of trespass since joining the force.

He said he believed the stretch of line near Eastbrook railway station in Dinas Powys is one of the worst places for trespass, and has heard of at least three deaths near one spot on the line in the past year.

Although children have often been seen on the line, the main culprits were adults, who had chosen not to use the overhead footbridge, he said.

"I've seen mothers with babies in pushchairs, and they just pick up the pushchair and walk straight across the track," said Pc Henderson.

"I've seen them not even looking before they walk across."

Pc Henderson said incidents of vandalism were common on tracks throughout Cardiff and the south Wales valleys.

He said stones were thrown at trains, and objects like ballast and concrete ducting had been placed on the track - with the potential to cause derailment and serious injury.

In one serious incident in Swansea, vandals put objects on the track which burst through a train's fuel tanks.