The Liberal plan for massively improved commuter rail service is estimated to cost as much as $12-billion, a total that would soak up the majority of money available for Toronto-area transit over the next decade.

The figure emerged after a board meeting Thursday by the regional transit agency Metrolinx, where expanded GO service was characterized as a game-changer that could push the need for a relief line in downtown Toronto into the indefinite future.

Toronto Transit Commission chief executive Andy Byford countered, though, that a downtown relief line remains a priority for his agency. He could not be reached for an interview Thursday afternoon, but he confirmed in a text message that the proposed line's importance remains even if GO service is improved.

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Funding it, though, may prove a difficult matter. With $15-billion of provincial money promised for transit in Toronto and surrounding communities over 10 years, the huge bet on GO would seem to leave only crumbs for other priorities.

However, Metrolinx chair Rob Prichard clarified later that the final tally for GO expansion could drop as the budget is refined. Some parts of plan will be outside the Greater Toronto and Hamilton area, he noted, meaning that those portions of the bill might not fall within the $15-billion available. He added that municipalities may pick up a share of the bill.

However, the sheer scale of GO spending, and its prominence within the overall budget, stands as a stark shift from just a year ago. As recently as last spring, the next generation of priority Metrolinx projects were pegged to cost $22.6-billion, with roughly one-third of that going toward GO-related improvements. The current plan has a shorter time frame and a smaller package of funding but earmarks more dollars for GO, with the tentative budget for commuter rail equal to roughly three-quarters of the available money.

Improving GO by electrifying the lines – which allows trains to run closer together because they can accelerate and decelerate more quickly – was championed by former transportation minister Glen Murray. He dubbed it Regional Express Rail and painted a picture of trains running every 15 minutes across the region.

The idea was adopted by Toronto mayoral candidate John Tory, who piggybacked his "surface subway" proposal onto the provincial plans, adding a spur on Eglinton Avenue West and calling it SmartTrack.

On Thursday, Mr. Tory pointed to a Metrolinx report tabled at the board as evidence that his plan could be done in seven years. The report says the entire network can be complete in 10 years and he said he would push hard to have the lines on which his plan relies done toward the start.

"I just believe the case for doing this, because it provides so much help to so many people in 416, where the problem is most acute, will commend itself to them," he told reporters. "I'm confident that when they have to prioritize, because you have to pick lines to do one at a time, that they're going to pick these lines among the first to do."

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According to Metrolinx chief executive officer Bruce McCuaig, though, it is still not clear if the lines will be done sequentially.

"We need to go through a lot of business case planning and staging planning to get to what the final approach to electrification's going to be," he said.

"We plan to come forward with that service plan to the board in September. One of the pieces we'll be looking at is what is the right kind of sequence? Is it corridor by corridor? Is it putting together pieces of corridor, depending on different kinds of service models? Those are kinds of questions we're going to be working on."