Toronto's transit agency says it is investigating after a video posted to YouTube last week showed people riding on top of a rapid transit train.

The Toronto Transit Commission says it has pulled surveillance video from inside its vehicles and stations as it investigates what took place on one of its Line 3 trains.

Spokesman Stuart Green says such stunts can be "incredibly dangerous and potentially fatal," while also potentially disrupting service.

The YouTube video shows at least two masked people riding on the roof of a moving rapid transit train, waving Canadian flags.

Green says the transit agency could issue a bylaw fine of $425 or pursue more serious criminal charges for those involved, depending on the investigation's outcome.

Toronto police say they are aware of the incident and the TTC's investigation but declined to comment further since no criminal charges have currently been laid.