A new underground transit line into downtown Toronto will "absolutely" still be necessary regardless of what is done to increase use of the GO corridors, TTC head Andy Byford said Wednesday.

The warning came a day after mayoral candidate John Tory threw his backing behind a provincial proposal for upgraded service on GO tracks, saying it was his version of a downtown relief line. Although he did not explicitly rule out a tunnelled DRL, as traditionally envisioned, he called it "the wrong line" and suggested his plan for surface transit would push that project to an indefinite future.

But it may not be that easy. Although he was careful not to comment directly on Mr. Tory's plan, Mr. Byford made clear that the long-planned underground DRL is key to the city's transit future.

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"We at the TTC have always advocated better use of the GO lines, they're a very useful resource," he said after the monthly TTC commission meeting.

"Ultimately, any additional capacity is welcomed in the city. We all know that the TTC is carrying greater rider numbers than ever. That number is predicted to rise up to a point in 2031 where the southbound Yonge line will be completely overwhelmed. So for that reason, we say that… a relief line is necessary. If, in the interim, other measures can be undertaken to address congestion … then we fully support that."

A roughly U-shaped downtown relief line, connecting the core with the Bloor-Danforth line, has been mooted for decades. Without this additional capacity, the Yonge subway and its interchange station at Bloor Street are projected to face crisis levels of crowding in less than two decades.

The eastern portion of the DRL is more urgently needed and the city's planning and growth management committee will vote Thursday on an accelerated environmental assessment. This would go to full council for approval next month but, if either vote fails, the project will not be revisited until next year.

A tunnelled DRL will invariably take at least 15 years. In backing electrified, high-frequency service on GO corridors, Mr. Tory sought to accelerate the timeline dramatically, promising completion within seven years. But Mr. Byford made clear that one line does not remove the need for the other.

"Ultimately we still believe – all the figures show and all of our studies show and the downtown relief study that we did some time ago still shows – that whatever you do, there's still a need to add capacity in that critical corridor to relieve Yonge and Bloor station and to relieve pressure on the Yonge line," he said. "That hasn't gone away, that remains our number-one priority."