The TTC is investigating after a video surfaced online showing a moving subway car with one of its doors open last night.

The video, which was taken Friday night from inside a subway car on Line 2, showed a train speeding over the Bloor Viaduct with one of its doors wide open.

TTC spokesperson Brad Ross confirmed that at around 6 p.m., crews noticed an issue with the doors of an eastbound train at Castle Frank Station.

"It was investigated by the crew and I can only assume they felt they had resolved it but clearly it hadn’t been resolved. We see the video of the train passing over the viaduct with the doors wide open," Ross told CP24 Saturday.

"When that train got to Broadview Station, it was again experiencing those door troubles, obviously, and the train was then taken out of service and brought into the yard."

Ross said there was no indication that the crew knew the doors were open while the train was in motion.

"It is an incredibly serious situation and we are treating it very, very seriously and investigating. The train is in the yard now and our technicians and mechanics are going over that train with a fine tooth comb to understand exactly what happened," Ross said.

"Whether it is over the viaduct or in a tunnel it is incredibly dangerous for doors to be open like that."

As part of a safety feature built into the trains, Ross said emergency brakes should kick in if the doors are open.

"That didn’t happen here so we need to understand why," he added.

Ross said that while it is very rare, if a person does see doors open while a train is in motion, they should press the yellow emergency alarm.

The transit agency has not ruled out tampering as a possible cause for the malfunction, Ross said.