WINNIPEG, Manitoba (Reuters) - A Canadian Pacific Railway train was parked for a change of crew when it unexpectedly rolled down a steep embankment in British Columbia’s Rocky Mountains on Monday, killing three crew members, a transport regulator said.

The train, hauling 112 cars of grain, was parked for two hours at the last station before a tunnel near Field, B.C., to allow a new crew to replace one that was near its maximum hours of service, Transportation Safety Board (TSB) senior investigator James Carmichael said on Tuesday.

He said emergency air brakes were applied before three crew members, a locomotive engineer, conductor and conductor trainee, boarded the train and prepared to depart for Vancouver.

The train then “began to move on its own,” exceeding its maximum track speed of 20 miles (32 km) per hour for the tight curves and steep mountain grade, and derailed, Carmichael said.

The area features some of the most challenging terrain for trains in North America, he said.

Canadian Pacific could not be immediately reached. On Monday, CP Chief Executive Keith Creel said in a statement that the company would not speculate on a possible cause.

Carmichael said the TSB’s investigation would continue to determine how the loss of control happened.

Eight railway workers have died in Canada since November 2017, including Monday’s deaths, according to the Teamsters union that represents Canadian rail workers.