The boys, aged 16 and 17, are also alleged to have taken pictures on a mobile phone of a concrete slab and a metal bar laying across railway lines in Stanground, Peterborough.

One passenger on the Central train from Peterborough to Stansted Airport on October 22 last year, told police of fearing he was going to be killed when the train hit the obstacles.

Peterborough Crown Court heard yesterday that the incident could have caused a major disaster as, after colliding with the nine-and-a-half stone slab and running over the 10ft metal bar, the train came close to plummeting off Black Bridge, into the River Nene.

The two defendants both pleaded not guilty to endangering the safety of railway users with intent, but on a charge of endangering the safety of railway users, the 17-year-old pleaded guilty while his co-defendant pleaded not guilty. They cannot be named for legal reasons.

Prosecuting, Ebraham Mooncey said: "The Crown says it was a joint enterprise – they both acted together. We don't have CCTV of what took place or know how the items came to be upon the railway track, but we know they were both there.

"These two were at Black Bridge, and at about 5.55pm, a train travelling from March to Peterborough hit an obstruction. The concrete slab shattered, causing some damage to the train, and the train went over the metal strip.

"Fortunately, other than being shaken up, none of the occupants were injured. There was a risk it could have derailed and greater disaster could have resulted."

He claimed it wasn't the first time the two teenagers had done something like this: "In the past, they have put smaller items on the line, and the only intention they could have had was to cause a greater impact with their activities."

The jury heard that, in a statement, train driver William Chamberlain said he saw "something alarming in the distance" and had slammed on his brakes at 60mph, but seconds later, the train collided with the slab and metal bar.

He had felt a huge jolt and had prepared himself for a derailment, but the train just shuddered to a halt.

In a statement, guard Nigel Clennett said: "I felt a violent shudder. It was strong enough to throw me off balance and I had to grab a seat.

"There was a loud crunching and grinding noise and I thought 'oh my God, what's going on?' It occurred to me how lucky we had been and how the train could have ended up in the river."

Passenger John Bradshaw said: "There was an awful, loud tearing noise. It was a horrendous sound. I thought the bottom had been ripped off. I was shocked and scared. I thought we could have been derailed and killed."

The driver cleared the line and the damaged train limped back to Peterborough station.