The TTC says it is investigating a so-called 'subway surfing' video recorded on Line 3 Scarborough that was posted to YouTube on Canada Day.
"We take incidents like this seriously and will prosecute to the full extent of the law," said TTC spokesperson Stuart Green in an email.
"Stunts like this are incredibly dangerous and illegal."
The four-and-half minute video shows several people riding atop Line 3 rail cars as trains speed between stops. There are also shots that appear to have been recorded by a drone-mounted camera.
The video was published on YouTube by user ChaseTO on July 1.
"Celebrating the greatest country on earth by surfing trains in my home city," the video description says.
Green said the TTC has pulled security video from its own trains and stations as part of the investigation.
These kinds of stunts can "lead to serious injury or death as well as service delays and massive inconvenience for our customers," he said.