Toronto's city planning department is taking over and broadening the environmental assessment for the Scarborough subway extension after "difficult and quite contentious discussions" with the TTC, leaving the transit agency to focus on construction.

The shakeup in how transit is planned and built in the city will be the model for future projects, according to Toronto Transit Commission chief executive Andy Byford.

Chief planner Jennifer Keesmaat characterized the new approach as the difference between simply "laying down infrastructure" and using transit to create "great thriving prosperous places."

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The shift comes after concern on the part of some city staff that the TTC was moving ahead too quickly amid the uncertainty – with two of the five major mayoral candidates vowing to revert to the original plan for light rail – that continues to surround the Scarborough transit plan.

This latest twist in Scarborough, which should not affect the work's overall timelines, also offers a rare glimpse into the project. Progress, which is still at a very preliminary stage, had been delayed by the dust-up over division of responsibilities. Now that that issue has been settled, a request for proposals for initial engineering work is expected to go out this week.

The decision to give city planning the lead on the EA – which will include determining the route and where and how many stations there will be – came after sharp differences of opinion between that department and the TTC over how best to proceed with the project.

"They were difficult and quite contentious discussions," said Mr. Byford, who acknowledged that not everyone on his team was pleased to see city planning assume the new role.

"I have to see the bigger picture. And it seems to me that the city does have a point in saying you can't just look at projects as purely transit projects, you've got to look at what the wider implications for the city are. I'll be honest with you, there's also a bit of me that says we've got quite enough on our plate – and that is the understatement of the year."

The city planning-led EA will look beyond the role of transit as a mechanism purely for moving people.

"Part of the lens that city planning is going to bring to this exercise is really that city-building lens, that place-making lens," Ms. Keesmaat said.

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"This change will ensure that we're integrating our considerations for new lines into a network-based approach and also integrating the transit alignment policies with our city-building policies around densification and transformation of urban environment."

Rapid transit, particularly in Scarborough, has shaped up as a key election issue in this fall's mayoral election. Light-rail supporters say that form of transit is cheaper, faster to build and serves more people. Subway boosters insist underground transit is best and prefer to frame the debate as one that has been settled.

Some high-ranking transit officials will admit privately that the subway extension is far from a sure thing, with one saying it would likely be the next city council that decides on the project's future. Another pegged its chances at "60-40 or 75-25."

The long-debated project continues to face hurdles, among them that the original legal agreement for an LRT in Scarborough remains in force. In a recent e-mail, Metrolinx spokeswoman Anne Marie Aikins said that changing that agreement is an ongoing process. Among the difficulties are negotiations over how to divvy up sunk costs.

Another issue is securing the money needed to pay for the controversial project.

Then finance minister Jim Flaherty promised $660-million from the Building Canada Fund last fall, subject to the normal approvals process. Earlier this spring, Mayor Rob Ford revealed that the city had not yet completed its business case or application for the funds, a process that is still under way.

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Subway supporters also will have to win a series of big-dollar funding votes at council as the project proceeds over the next few years. If the next mayor is a supporter of the project, that mandate will help carry those votes. But a lot of work remains before the proverbial shovels can go in the ground.

"This will be debated for a long time," subway proponent Councillor Glenn de Baeremaker, who represents a ward through which the subway would run, predicted in a recent interview.

"Will there be a guerrilla war waged [at council] for the next four years? Yes, unfortunately there will, and we'll waste a lot of time and energy and effort focused on attacking each other and sabotaging each other's plan, instead of working together in co-operation and building something really truly spectacular. But in the end, something truly spectacular will be built in Scarborough."