An inquiry was under way last night after four railway workers were killed by a runaway wagon.

The wagon, weighing several tonnes, became detached from a train in a yard further up the track on the West Coast Main Line in Cumbria, and began rolling downhill, gathering speed, police said.

People at the yard tried to warn the workers four miles below but the message did not reach them, British Transport Police said.

A spokesman said: "The alarm was raised. The people at the depot where the trolley started moving from tried to warn workers below but they didn't hear."

The wagon travelled out of control for four miles before ploughing into a gang of ten workers unaware of the danger and working in poor light further down the track.

There was no lookout because the line had been closed while the work took place, investigators said.

A British Transport Police (BTP) and Health and Safety Executive (HSE) investigation will centre on how the wagon became detached from the train before the accident at 6.10am yesterday at Tebay, north of Kendal.

Besides the four killed, three others, all in their 30s, suffered leg injuries.

The most seriously injured man underwent an operation and was in a stable condition, along with another man, in hospital. The third man was discharged after treatment. Three other workers escaped injury and were interviewed by British Transport Police.

The wagon, described as a trolley used for carrying rail lines, stopped a mile down the track after hitting the men.

BTP Acting Superintendent Peter Davies said: "The scene is one of devastation.

"The four dead suffered severe injuries and would have died instantly. We are trying to find out what the cause of this tragic accident was.

"A flatbed rail wagon of quite considerable tonnage and somewhere in the region of 12ft to 15ft in length was loaded with redundant rails.

He said: "What we don't know is why this wagon started its journey downhill. It was a runaway train gathering speed, and we are looking at quite a considerable speed, anything up to 30 or 40mph when it hit the workers.

"They were working in the dark and they probably had their safety gear on and there would have been no noise."

Mark Lenderyou, an inspector with the HSE, said: "There would not be a need for a lookout in this sort of work because both lines were blocked to the passage of trains. Part of the investigation is to discover how the trolley became uncoupled and rolled down the track.

"If the brakes are on, a trolley usually would not move. We are investigating if there was a fault in the system, how the job has been organised, was there a problem with the brakes and was there something else wrong with the machine."

The dead workers were from the area. They were working for Carillion Rail, a sub-contractor upgrading the West Coast Main Line to accommodate the new tilting 150mph "Pendolino" trains.

The line was closed after the accident and will reopen at 6am today.

All the victims were members of the Rail, Maritime and Transport Union, which has called for a public inquiry and said similar engineering work should now be suspended.

A 40-year-old man, who had been working further up the track, said: "It would hit them like 20 tonnes of death straight out of the dark.

"They would have had no idea it was coming."

The worker, who did not wish to be named, said: "This was the worst nightmare for any rail man - an unmanned wagon coming straight at you out of the pitch darkness.

"They would have known nothing, heard nothing, seen nothing, until it was all too late. They were working with their lights on, concentrating on their jobs.

"No one believes this, but you really can't hear a train coming, let alone a wagon travelling under the momentum of its own weight."