OTTAWA, Ontario (AP) -- Passengers screamed "Stop! Stop!'' seconds before their bus crashed through a crossing barrier and into a commuter train during morning rush hour in Canada's capital on Wednesday, killing six people and injuring 34.

"He smoked the train," witness Mark Cogan said of the bus driver, who was among those killed. "He went through the guard rail and just hammered the train, and then it was just mayhem."

It was not immediately clear what caused the bus to smash through the lowered barrier at a crossing in suburban Ottawa.

The front of the double-decker bus was ripped away by the impact, and the train's locomotive and one passenger car derailed, though there were no reports of major injuries to train passengers or crew.

Eight were still listed in critical condition late Wednesday. The crash brought trains on the national Via Rail's Ottawa-Toronto route to a standstill.

It was Canada's second major rail accident in less than three months. A runaway oil train derailed and exploded in a Quebec town on July 6, killing 47 people in the country's worst rail disaster in more than a century.