The horrific collision sheared off the front of the bus and knocked the Via locomotive and one of four passenger cars off the tracks.
Six people died, one of them the bus driver, while several area hospitals were flooded with more than 30 patients, including eight who were still listed in critical condition late Wednesday.
"People started screaming, 'Stop! Stop!' because they could see the train coming down the track," said Carleton University student Tanner Trepanier, who was in the upper level of the new, double-decker OC Transpo bus.
Both levels of the front end of the bus, extending back to the driver's seat, were ripped off by the impact.
"There was a lot of screaming, but then people were actually relatively calm, considering the situation," Trepanier said of the aftermath.
Alex Begin, on his way to his job in downtown Ottawa, was midway back on the lower level of the bus and said the driver hit the brakes only after passengers started yelling warnings.
"Oh yeah, we went right through the (safety) barrier," said Begin, who was at a loss to explain the driver's apparent lapse of concentration.
"I don't know, too early in the morning? He just wasn't fully conscious or something," said Begin. "Because until people started yelling, I guess he didn't even realize what was going down."
Romi Gupta, a 40-year-old downtown office worker, boarded what she called the "overfull" bus at its last stop before the crash.
"The driver was OK; I got in a minute before and I said hello to him and he was fine," said Gupta.
She, too, could see through the bus windows that the train was coming.
"The bus was too fast I find, he could not put the brakes on."
The head of the transit union local confirmed the driver was among the dead.
In an email Wednesday night, Craig Watson of the Amalgamated Transit Union identified the 45-year-old driver as Dave Woodard.
Peyman Shamsi, a friend of Woodard who had started with OC Transpo 10 years ago, said he was "one of the nicest guys" at the bus company.
"I couldn't even say a bad thing about him. I'm really shocked, I don't know what happened," said Shamsi, who has driven the same route as Woodard did.
"I'm surprised because he was a safe driver."
Woodard leaves behind his wife Terry, who had celebrated a birthday on Tuesday, and teenage stepdaughter.
A number of people gathered on the tracks near the crash site Wednesday evening for a candlelight vigil.
The bus was on a dedicated transit line that runs parallel to a busy commuter artery just east of a suburban Via Rail station, about 10 kilometres west of Parliament Hill.
Witnesses on the ground said the northbound bus simply didn't stop, despite flashing lights and lowered safety barriers warning of the westbound train which had already stopped commuter traffic nearby.
"Boom! It went into the train like that," said Pascal Lolgis, who watched the bus smash through a safety barrier.
"He just didn't stop. He just keep going like that. Then he got hit."
Another witness, Mark Cogan, also said the safety barrier was down.
"I just thought maybe there's a side way around or something but instantly he just ... he smoked the train," said Cogan. "He went through the guard rail and just hammered the train and then it was just mayhem."
A broken safety barrier was visible under the bus carriage.
The Via locomotive and one of four passenger cars came to rest askew on the tracks, but Via officials said there were no injuries among the more than 100 passengers aboard. The company suspended its Ottawa-Toronto service as a result of the crash.
Rescue crews swarmed over the mangled bus wreckage as ambulances, fire trucks and police vehicles flooded the scene, rushing the injured to area hospitals.
"This is one of the worst events we have seen in the city or at this hospital; the tragic injury and loss of life to people," said Dr. Andrew Falconer of the Queensway-Carleton Hospital.
"It is overwhelming to imagine the suffering that is going on. We have had other mass casualties (events), but not for many years. This one is definitely tragic."
The accident occurred just before 9 a.m. and a number of students from Algonquin College and Carleton University were aboard the bus.
A trauma centre for worried families was set up at a nearby sports complex; by midday, fearful-looking people were arriving in small groups.
Lee Tran, however, had to leave without learning the fate of a family member who he said routinely rides that same bus daily to Carleton University.
"We just have to wait," Tran said.
Abera Feyissa, meanwhile, received good news. His Grade 12 son was on the bus and on his way to his job downtown. Feyissa managed to reach his son by cellphone at a local hospital, unhurt.
"It's a shock," Feyissa said as a police officer guided him to his car.
Prime Minister Stephen Harper issued a statement calling it "a tragic morning in the nation's capital."
"On behalf of all Canadians, Laureen and I extend our thoughts and prayers to all those affected by this tragedy," said the prime minister.
The Transportation Safety Board dispatched 11 investigators to the scene to start what a senior board official said will be a very complex job that could take months.
Jean Laporte, the board's chief operating officer, said the team will be thorough.
"We are documenting, photographing the wreckage and the accident site," Laporte said. "We will be assessing the crossing, its design and the sightlines, we'll be checking the warning systems and the gates at the crossing to ensure they were functioning correctly."
Data from the onboard locomotive recorder will also be reviewed, "as well as from other recording devices that might be available," Laporte added.
The safety board said on its website that there were 2,162 crossing accidents on federally-regulated railways between 2003 and 2012, resulting in 266 deaths and 346 serious injuries.
Since Ottawa introduced the double-decker buses just over a year ago, drivers have voiced concerns about brakes and the top-heavy nature of the vehicle.
But OC Transpo's general manager, John Manconi, rejected the suggestion Wednesday that the buses might not be road-worthy.
"I can assure you if OC Transpo knew of any incidents that would make any vehicle unsafe, we would not be putting it in service," Manconi said.
— With files from Murray Brewster, John Ward, Jennifer Ditchburn and Steve Rennie
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