Lessons have not been learned by Network Rail following the Tebay rail tragedy earlier this year, the RMT union claims.
Four people died in the Tebay rail tragedy in February
General Secretary Bob Crow says people could have died in two runaway vehicle incidents at the same site in Stockport on the 5 and 17 August.
Four people died when they were struck by a runaway truck in Tebay, Cumbria, in February.
Network Rail says it treats safety seriously and an inquiry has begun.
The Stockport site is part of the West Coast Main Line renewal project, and involves a number of contractors and sub-contractors.
Mr Crow said: "It is shocking to learn so soon after the deaths of four of our members in the Tebay disaster that there have been two more runaway incidents this month in Stockport.
"What is even more disturbing is that the incidents appear to have happened in similar circumstances on the same site and could so easily have claimed more lives."
Mr Crow said Network Rail should stop employing private contractors and sub-contractors for repair work and should instead carry out the repairs itself.
He added: "After the Tebay tragedy we called for an inquiry into safety standards in the fragmented, privatised rail industry.
"So far we have had no inquiry, but the runaway incidents have kept on happening. It seems that safety is still taking second place to profit."
Mr Crow said he had written to Network Rail about the issue and said the first Stockport incident had not been mentioned at a recent meeting of the Safety Council.
A Network Rail spokesman said the recent incidents involved a small piece of engineering equipment called a road-rail vehicle and had happened during daylight hours at slow speeds.
No-one was injured in the incidents.
He added: "Any safety related incident on the railway is treated extremely seriously and a full investigation is underway. Initial findings point to operator error as the cause.
"Since the Tebay tragedy in February, Network Rail and its contractors, with help from trade unions, have undertaken a huge amount of work to make the railway a safer place for our workforce."
The men who died in the Tebay tragedy were Chris Waters, 53, of Morecambe, Lancashire, Colin Buckley, 49, of Carnforth, Lancashire, Darren Burgess, 30, of Carnforth, and Gary Tindall, 46, of Tebay.
Two men are still on bail as part of a police inquiry into the deaths.