Streetcars on the 502 Downtowner and 503 Kingston routes have been pulled out of service because the pneumatic lines on the vehicles' brakes and door systems were freezing up.
The TTC says some of the streetcars on 511 Bathurst route have also been affected.
TTC spokesperson Jessica Martin told CBC News that cold weather causes moisture in the streetcar brake lines to freeze, forcing them out of service.
The TTC had similar problems yesterday.
The TTC is operating buses on the affected routes.
Doors and brakes on the new streetcars, which are slated to enter service over the next few years, operate using electronics and don't have the same problem in cold weather.
The TTC currently has three of its new streetcars operating on the 510 Spadina route.