The Toronto Transit Commission is investigating what it calls "an incredibly serious situation" after video surfaced online showing subway train doors stuck open as the vehicle travelled over the Bloor Street viaduct.

A passenger shot the video while on a Line 2 car around 6:30 p.m. Friday.

<a href="https://twitter.com/CityNews">@CityNews</a> TTC tried to fix door of a subway Car on line 2. Then TTC let the train go from Castle Franck to Broadview <a href="https://t.co/Er0YC7ysr5">pic.twitter.com/Er0YC7ysr5</a> —@NEWSFORWORLD

The passenger said the train doors remained open as the car travelled between Castle Frank and Broadview stations.

TTC spokesman Brad Ross said that normally an open door on a moving subway would trigger a safety mechanism that would alert the driver and stop the train.

But the driver had taken the doors offline after an earlier malfunction, Ross said, so the emergency brakes did not activate.

"When it pulled away from the station, the inertia and the propulsion of the train caused the doors to freewheel open," Ross said. "They hadn't been properly locked, from what we can tell."

Ross said the door should have been both locked and barricaded after it was taken offline.

"We need to interview the crew on Monday and get their take on exactly what procedures they followed or thought they may have followed because this is an incredibly serious situation," he said.

"We cannot have train doors open, obviously, when the train is in motion. Whether it's in a tunnel or crossing a bridge, it doesn't matter — it's very, very unsafe."

Ross said the TTC will have more information on Monday.