Passengers involved in a fatal collision between a Via Rail train and Ottawa city bus described a scene of panic and horror as investigators attempt to piece together the circumstances of the tragic accident.

At least six people are dead and dozens are injured after the train and double-decker bus collided shortly before 9 a.m. ET.

Passengers who were aboard the OC Transpo bus were reported to have been screaming at the driver to stop as the vehicle drove through a marked barrier at a level crossing in the city’s southwest end.

"(People) were shouting 'stop, stop' and then as I looked up, the bus was coming into contact with the gate," bus passenger Rebecca Guilbeault told CTV News in an interview at the site of the crash.

She said the front of the bus “impacted at once” and the passengers "flew" forward.

Guilbeault, who was riding the bus with her one-year-old son, said she wedged herself between "piles" of people and her child.

"There was dust everywhere and I all remember was I grabbed my son and ran off the bus," she said in tears.

Mahima Sharma said she takes the 76 bus every day and recognizes a number of the others as "regular" passengers.

"I remember looking out the window and seeing the train coming and seeing the cars on the other side of the road that had all stopped, and I was just wondering in my head 'why are we not stopping, why are we not stopping,' ” she recalls.

"Someone yelled on the bus, that's when, I'm pretty sure, we slowed down."

Sharma, 26, said she used to enjoy watching the trains pass by during her morning commute.

"I might not look forward to seeing the trains every day," Sharma told reporters on her way out of an Ottawa hospital. "It's hard to say."

All the casualties from the crash occurred on the bus, officials said. There were passengers on the Via train at the time, but no injuries have been reported.

A passenger who was sitting on the right side of the train told CTV News Channel in a phone interview that he felt a "sudden bump" at the time of the crash. "There was a lot of dust."

He said passengers on the train didn’t panic, but that many of them were aware that a collision had occurred.

In the aftermath of the collision, part of the stairway leading to the upper deck of the bus was reported to have been torn off, but many people were able to scramble off the vehicle.

Photos of the scene show the front of the bus completely sheared off from the impact of the crash.

Pascal Lolgis, who witnessed the crash, said the bus appeared to drive through a lowered crossing barrier.

"He didn't stop. He must have lost his brakes," Lolgis told The Canadian Press.

Another witness Mark Cogan also said the rail barrier was lowered at the time of the crash.

"The train is going through," Cogan told CTV News. "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."

Many of the passengers on the bus were on their way to work or school at the time. Amanda Brook said she was listening to her iPod when she saw the train approaching.

"Everyone was yelling ‘stop, stop’ to the bus driver," she told CTV News Channel.

Brooks said she then felt the bus attempting to stop. "The bus started shaking," she said. "I thought the bus was going to fall over."

who was on the lower level of the double-decker bus, said she was thrown into a group of passengers when the crash occurred and injured her foot.

“I could see people dying,” she said. “There was a person who was just near me, I saw his head out of the front door, just his head hanging out.”

With reports from CTV Ottawa