The radical rerouting is necessary, the TTC says, to allow crews time to repair the "severely deteriorated" tunnel roof between Bay and St. George stations.
"That work has to be done, and it cannot be done with trains in operation," said Mitch Stambler, the Toronto Transit Commission's manager of service planning. "So we have to circumvent the affected part of the tunnel."
The detour will see Bay station closed, on Saturday and Sundays only, from Feb. 18 to March 31. Eastbound and westbound Bloor-Danforth trains will bypass the station and turn south, terminating at Museum.
Passengers who want to continue west or east will have to change trains at Museum, and TTC staff will be on hand to answer questions about what train to take, Mr. Stambler said.
The trains will travel on a little-known track connection between the city's two major subway lines, called in rail terminology a "wye," which was used briefly for passenger service when the Bloor-Danforth line opened in 1966. The extra tracks are now usually reserved for out-of-service or work trains. This type of rerouting would be impossible elsewhere on the system.
Westbound trains will pass quickly through "Lower Bay" station, which is no longer used except as a movie stand-in for stations in other cities.
Scenes in the 1994 science-fiction film Johnny Mnemonic, starring Keanu Reeves, were shot there. Subway trains will not stop at this phantom station.
TTC officials say the unusual rerouting will save passengers the inconvenience of having to switch to shuttle buses, which cannot handle subway passenger volumes.
On the affected weekends, trains across the system will operate less frequently, however -- every six minutes instead of every four to five minutes -- a change the TTC says is required because both routes will be using the same tracks going into Museum. The lower level of St. George station will also close, and all trains in and out of the station will use its upper tracks.
The TTC says there will be major structural repairs of the roof of the Bloor-Danforth subway tunnel, almost directly under the Park Hyatt Hotel at Bloor Street and Avenue Road. The existing roof must be replaced.
The demolition will take place over three weekends; the other weekends will be used for preparation and cleanup.