UNION leaders called for a full public inquiry yesterday after four workers with a private maintenance company were killed by a runaway train wagon.

It travelled four miles and hit them at 40mph on the west coast main line. Three others were injured.

Investigations were focused last night on how the wagon, weighing several tons, became detached from a train further up the track at a yard before rolling downhill and ploughing into a gang of 10 workers at 6.10am at Tebay, north of Kendal.

The Health and Safety Executive said it would also be investigating why brakes did not come into action and stop the trolley.

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

There were added concerns that the team of engineers had no lookout, although investigators said this was understandable as the line was closed while the work took place.

Mark Lenderyou, an inspector with the HSE, said there would not be a need for a lookout 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,'' he said. ''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.''

Peter Davies, acting superintendent of British Transport Police, said the scene was ''one of devastation''.

He added: ''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 40 mph when it hit the workers.

''The men were working in the dark. They were using cutting and grinding equipment on the track, so there was a lot of noise and they didn't hear it coming. The four dead suffered severe injuries and would have died instantly.''

He said people working at the Scout Green depot at the time the wagon broke free were being questioned by detectives. Three other workers who escaped injury were also being interviewed.

He said: ''We don't know what was going on at Scout Green depot at the time. This unit was there and became detached but we do not know why.''

The dead workers, who have not been named, were locals. All were members of the Rail, Maritime, and Transport Union, which wants a full, independent inquiry.

Bob Crow, general secretary of the RMT, said: ''We want a public inquiry into this tragedy rather than an investigation by the Health and Safety Executive. Pending this inquiry, we want to see Network Rail suspending work in similar terrain until the causes of the tragedy are clear.''

Phil McGarry, RMT Scottish organiser. ''I would echo these sentiments. No stone should be left unturned to make sure this does not happen again.''

Peter Rayner, a rail safety expert, said much maintenance was not done by dedicated railway workers. He said: ''One of the issues that an inquiry will need to look at is to ensure the contract, and sub-contract and sub-sub-contract culture in the industry is not the root of this accident. Commercial considerations should not be allowed to compromise safety.''

The engineering gang were working for Carillion Rail, a sub-contractor upgrading the line to accommodate the new tilting 150mph Pendolino trains. Carillion is one of the leading rail infrastructure companies in the UK, with more than 4000 staff and an annual turnover in excess of (pounds) 200m.

A spokesman for Network Rail said: ''Our thoughts are with the families of the dead and injured.''

The line will reopen at 6am today.