Vandals put a set of football goalposts on a railway line in Oxford, bringing a London-bound express train grinding to a halt just 24 hours after the Potters Bar tragedy, it was revealed today.

The 1.45pm Thames Trains service between Banbury and Paddington on Saturday was accelerating at Hinksey after leaving Oxford Station when the driver spotted the obstruction across the track.

He immediately hit the brakes but could not prevent the three-carriage train from hitting the metal posts.

The train, which was travelling up to 50mph, stayed upright on the rails, although the debris became wedged between the front bogie and axle.

An oil filter beneath was damaged but the train continued its journey about 30 minutes later. The incident delayed 10 trains.

Thames Trains spokesman Jonathan Radley said: "We are very alarmed and extremely concerned that anyone would be so reckless. This was just over 24 hours after Potters Bar and that's horrific.

"We think that those responsible were local and we would urge parents to ensure that their children know not to play on the railway lines or leave objects on them."

British Transport Police believe the five-a-side-style posts, measuring 4ft by 3ft, had been taken from a sports ground near the railway line. At first they thought the posts, complete with netting, had been hurled off a footbridge over the main line.

But according to police, the train guard later reported seeing three youths hiding in bushes at the side of the track, giving the impression they had deliberately laid the posts across the track and then waited to witness the result. A police spokesman said: "The driver saw the obstruction and jammed the brakes on but was unable to avoid hitting it.

"Anything laid across the line always brings with it the threat of derailment, and the fact this was done deliberately so soon after the Potters Bar tragedy is beyond belief."

Anyone with any information can call the police on 0118 9590950.

Seven people were killed at Potters Bar when a carriage left the track and hurtled on to the station platform. Loose or missing nuts from a set of points are being blamed for the tragedy.