The Toronto Transit Commission evacuated its Dupont subway station Thursday morning after a train broke down and filled with smoke.
Fire officials said a piece of metal fell off the bottom of a train and got lodged in the electrified third rail, creating sparks and smoke.
Police and firefighters responded to the station following reports of an explosion.
Toronto police confirmed that the incident was mechanical in nature.
Ten people were treated for smoke inhalation.
Subway service on the Spadina line was disrupted for about an hour while investigators determined the cause of the breakdown.