A shocked train driver narrowly avoided disaster after a group of children as young as six blocked a railway line with concrete and stones.

The four youngsters aged six, seven, nine and 11 piled the objects on a stretch of track in Laisterdyke as passengers travelled between Bradford and Leeds. The driver spotted the obstruction in the distance and managed to brake before hitting it.

The group of youngsters were caught by British Transport Police, but are not being named.

A Transport Police spokesman said the train could easily have been derailed.

"The consequences could have been very serious, resulting in injuries or even death," he said. "We cannot emphasise enough the dangers of children playing on the railway, not only for themselves, but for staff and passengers on the railway."

PC Mark Stokes, of Bradford British Transport Police, pictured, added: "This needs to be brought to the attention of everyone. The public are our eyes and if they see kids playing near the line, they must let us know. If they see anyone near a railway line, tell us as they shouldn't be there."

The incident happened at the rail and road junction known as Ducketts Crossing in Daleside Road, Thornbury - within days of the reopening of the Bradford Interchange to Leeds line which was closed for five months.

Notices had been put up in the surrounding area warning residents that the railway would be fully operational from May 21. And leaflets have been distributed around local schools.

A day later, another three youths were caught by officers after placing stones, wood, plastic and metal over 100 yards of track in the Wyke area of Bradford.

The group, including a 14 and 13-year-old, also smashed a railway warning sign.

All three have been interviewed and reported for offences of obstruction of the railway, criminal damage to a warning sign and trespassing.

British Transport Police said officers were constantly in contact with schools and parents to educate children about the dangers.

To report any incidents of trespassing or vandalism, call the British Transport Police hotline on 0800 405040.