The accident just outside a suburban station in the city's west end caused Via Rail to cancel trains on its Ottawa-Toronto route.
Witness Mark Cogan told C424 TV 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."
More top news
The Duchess of Cambridge has made her first public appearance since her pregnancy was announced.
The bookseller clearly knows a good PR opportunity when it sees one.
The animal charity has taken in 33% more abandoned animals in 2014, officials say.