Both a new subway line into downtown Toronto and Mayor John Tory's proposed SmartTrack will be needed to keep the subway system from overloading within 25 years, according to ridership projections in a new report for the city.

The findings paint a picture of the two proposed transit lines operating in a "largely complementary" way. But the new data also raise the stakes for the mayor. The project on which he ran for office has to have trains come every five minutes to play its necessary role in the city's transit future, and his plan would have less impact than a relief line unless they can be made to run that often.

Neither the cost nor the technical feasibility of five-minute SmartTrack service has been determined. The city and regional transit agency Metrolinx are hammering out a way to integrate Mr. Tory's proposal with the province's plans, with the final agreement some time off.

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Central to Mr. Tory's election campaign was a proposal to piggyback on provincial plans for more rail service on GO surface routes, saying there would be more local stops and promising people they would be able to ride on a Toronto Transit Commission fare. He said this would act as much-needed relief for the increasingly crowded Yonge subway. But critics said from the start his plan could not take the place of a new subway route into the core – the long-discussed relief line.

"The findings in this summary report make clear the importance of the relief line," Toronto chief planner Jennifer Keesmaat notes in an introductory memorandum to deputy city manager John Livey. "However, by 2041, only the combination of the relief line with five-minute SmartTrack service will bring the projected … ridership [down] to approximately the capacity of the line."

The report's findings have the potential to set the stage for a political battle over the best way to spend transit dollars. They will add weight to Mr. Tory's argument that both lines are necessary, with SmartTrack helping tide over the system in the shorter term and the relief line taking longer to be built. But with limited money to spend – and with varying effects on ridership depending on how the projects are built and operated – council may have to decide which combination of the two would best serve the city.

"It's not an either/or scenario. We need both," mayoral spokeswoman Amanda Galbraith said in a statement. "We now have data confirming that the two lines complement one another and are both needed to reduce overcrowding on the Yonge subway."

Most of the figures in the new report are projected out to 2031. They all assume people will be able to ride SmartTrack for the same cost as the TTC, sidestepping the ongoing and thorny issue of regional transit-fare integration.

The modelling found the Yonge subway is projected to be overcapacity in 2031, hitting a peak-hour volume of 39,600 riders. That's an increase of about 14,000 over the current ridership, according to the TTC, though by then the line's capacity should have been raised to 36,000, the report says.

Adding the first phase of the relief line as proposed by the city – a tunnelled subway route running from Pape to the area of Nathan Phillips Square – would divert 3,600 people off the Yonge line at peak hour by 2031, leaving that subway at capacity.

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Having SmartTrack instead, but running the trains at only 15-minute intervals, would divert 2,300 riders by 2031, leaving the line overcapacity. Adding the relief line as well as that level of SmartTrack service, though, would take another 2,700 people off the Yonge line, for a total of 5,000 people.

If SmartTrack were to run every five minutes, it alone would divert 6,600 people off the Yonge line by 2031. Having the relief subway as well would cut another 1,800, for a total of 8,400 people.

"Contrary to much prior speculation, SmartTrack and the [relief line] are not significant competitors, or substitutes for, one another," University of Toronto professor Eric Miller, who oversaw the modelling, wrote in the report. "Both should be considered as viable additions to the transit network, subject, of course, to engineering and cost considerations."