Officials said 30 people were injured, 10 critically, at the peak of Ottawa's morning rush hour. The cause of the crash was not yet clear, said John Manconi with bus operator OC Transpo. Transit union president Craig Watson said the driver was killed.
It was Canada's second major rail accident this year. It comes after an oil train derailed and exploded in a Quebec town in July, killing 47 people.
Tanner Trepanier, who was on the bus, said passengers could see the train bearing down on them as they approached the crossing.
"People started screaming, 'Stop, stop!' because they could see the train coming down the track," Trepanier said.
Witnesses said the bus plowed through a lowered crossing barrier and into the side of the passing, four-car train. The locomotive and one passenger car derailed. Via Rail said there were no reports of major injuries to train passengers or crew. Via Rail canceled trains on its Ottawa-Toronto route.
Passenger Gregory Mech said the bus was approaching a train crossing where there is about a 90-degree bend. Mech said he didn't think the driver noticed that signals were flashing and that the barrier was down.
"The bus actually hit the train dead on," Mech told the Canadian Broadcasting Corp. "I could see that there were bodies on the train tracks. It was horrible."
Rebbecca Guilbeault, who was on the bus with her 1-year-old son, said she had put him in a stroller just before the crash and said people piled on top of him.
"I don't know if the bus driver blacked out. He didn't slow down, and it just impacted with the train," Guilbeault said. "I've seen a few people dead, someone ripped in half."
Witness Mark Cogan said the rail barrier was down.
"The train is going through," Cogan said. "And I was just looking around, just watching things happen. And noticed that in the bus lane, the double-decker bus ... I saw him, and he just kept going.
"I just thought maybe there's a side way around or something, but instantly, he just ... he smoked the train. He went through the guard rail and just hammered the train, and then it was just mayhem."
The train tracks, just outside a suburban train station, Fallowfield, cross a major city street and a transit line reserved for buses.
The national Transportation Safety Board's lead investigator, Glen Pilon, said Via Rail crossings have been a concern. Pilon said getting the black box recording is a priority to determine what went wrong before the crash.
Canada has seen 257 accidents involving passenger trains colliding with vehicles at level crossings over the last decade, the safety board said Wednesday.
"It is a tragic morning in the nation's capital, as a devastating accident between a bus and train has caused injury and death," Canadian Prime Minister Stephen Harper said in a statement.
Associated Press writer Rob Gillies in Toronto contributed to this report.
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