A runaway wagon that killed four track workers and injured three others was secured by just two small pieces of wood, rail union the RMT has claimed.
Investigators will look at how the trolley separated from its engine
The wood was jammed underneath the wheels of the wagon, which jumped over the blocks as lengths of rail were unloaded, it alleged.
The RMT said the claim backed up its call for a full public inquiry into the tragedy in Cumbria on Sunday.
Network Rail refused to speculate ahead of the official investigation.
The claims came after the wife of one victim said her husband had complained about safety equipment.
Christine Waters, 50, said her husband Chris, 53, feared new helmets were "more of a danger than safety" because workers could not hear when wearing them.
He was killed along with Darren Burgess, 30, and Colin Buckley, 49, from Carnforth, Lancs, and Gary Tindall, 46, of Tebay, Cumbria, when the wagon rolled downhill from a yard near Tebay and ploughed into them.
Reports suggested the men could not hear the wagon approaching.
General secretary of the RMT Bob Crow said information he had seen suggested there had been a "total lack" of safety management.
Five contractors were involved in the maintenance work, said the union.
"Our information is that the trolley was on the main line and not on a sidings," said Mr Crow.
"We have been told that it was detached from a
road-rail crane to enable the crane to unload it.
"Our information suggests that two two-inch blocks of wood were placed beneath the wheels of the trolley to stop it moving.
"Apparently loading had just commenced when the trolley crushed the wood and careered downhill."
Mr Crow said there had been claims the workers were not aware that there were colleagues on the
line down hill.
"If this information is correct our members will be outraged at the cavalier, reckless and disjointed approach to safety management and safe ways of working on the railways," he said.
Investigators will look at how the trolley separated from its engine.
On Monday it emerged Network Rail held a two-day inquiry last year into a similar incident.
A series of recommendations were made after a rail trailer was uncoupled from a vehicle, 20 miles from Sunday's tragedy.
Another rail vehicle caught it before anyone was hurt.
Network Rail said all the recommendations from that investigation had been adopted, leading to a new requirement that all trolley vehicles be
fitted with a braking system.
A spokesman said they were extremely concerned as to why the trolley in Sunday's incident failed to stop.
If the inquiry found a company or individual to be at fault, criminal charges could follow, said police.
TEBAY RAIL ACCIDENT
1. Wagon detaches from locomotive at Scout Green depot
2. Rolls down incline at speed of 40mph
3. Hits maintenance team killing 4 men and injuring 3 others
4. Continues down track for a further mile before stopping