Relief is in sight for TTC riders of the sardine subway on Yonge St.

But the breathing room won't last. In 15 years, the TTC will again reach a tipping point where the spine of its system won't be able to handle the crowds, according to a Yonge Relief Network Study by Metrolinx.

It shows that the GO electrification program, called regional express rail (RER), won't divert enough ridership from the Yonge line to negate the need for a separate relief line. Although Metrolinx hasn't studied SmartTrack separately, that finding isn't expected to change.

"There will be increments to SmartTrack — whether there are additional stations — but we don't consider that it would dramatically alter the impact because so much of SmartTrack is already embedded in RER," said chief planning officer Leslie Woo.

Mayor John Tory (open John Tory's policard)'s SmartTrack plan is based on adding more stations to the Stouffville GO line and building a spur west along Eglinton from the Kitchener line from Mount Dennis to near the airport corporate centre. Since much of the Yonge subway crowding comes from riders boarding to the north and east, that western spur wouldn't divert many riders, said Woo.

Regional express rail, which would run on the GO tracks around Toronto at frequencies of up to 15 minutes, is projected to reduce Yonge St. demand by about 4,200 rides in the peak period in 2025. By then the subway will have capacity for about 38,000 riders per peak hour in the peak direction.

The Yonge line south of Bloor is already operating about 11 per cent over its capacity of 28,000 passengers per hour. But the TTC's new computerized signaling system, automatic train control, which will allow it to run more trains closer together, will increase capacity by about 20 per cent when it’s up and running by 2021.

The new Toronto Rocket trains have already added breathing room to the subway, and the Spadina extension to York Region should divert more riders off the south end of Yonge when it opens in 2017.

Among five options for a relief line, three were found to have the most potential to relieve Yonge crowding:

A subway from Don Mills Station to St. Andrew had the best potential for offloading some Yonge riders.

A shorter version from Pape to St. Andrew would deliver "significant" relief to the Yonge and Danforth subways and eliminate the need to transfer at the overburdened Yonge-Bloor Station.

An underground "U" from the Danforth subway through downtown and back up to the Bloor line was found to deliver similar benefits to the shorter version but would be much more costly.

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While there's no funding for a relief line, it's critical that Metrolinx and the city be prepared, Woo said.

"We need to have projects always at all the various stages of development. This is about planning and being ready."

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