Forgive John Tory if he seems a little exasperated these days. Eager to move ahead with ambitious plans for Toronto, from reforming the police service to fixing up public housing to creating a grand new downtown park, the first-term mayor instead finds himself refighting an issue he had every right to consider settled: the fate of the Scarborough subway.

City council voted way back in 2013 to extend the Bloor-Danforth line deeper into the populous eastern suburb. Both the provincial and federal governments are on board with hundreds of millions in support. Mr. Tory ran for office in 2014 on a promise to push ahead, arguing it was time to stop talking and start building. Yet, three years later, backers and opponents continue to spar over the project.

They were at it again when city council's executive committee met on Tuesday to consider a new progress report. By Mr. Tory's reckoning, it is the ninth time councillors have debated the subway. Its foes, he said, are using everything but poison-tipped umbrellas and exploding cigars to kill it. Enough is enough, he says. Time to get on with it.

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He is surely right about that. Toronto has fallen decades behind other major cities. City councillor Chin Lee showed the meeting subway maps for cities from Singapore to Beijing to Kuala Lumpur that started building after Toronto. They now have webs of mass transit that put Toronto's to shame. Yet, Mr. Lee said, "Here we are arguing about one extension of a subway line."

One reason Toronto has lagged so badly is that politicians keep changing their minds. Transit plans are drawn up and approved only to be redrawn or thrown out, a vast waste of time and – because transit-building costs climb as the years pass – money. Over the past decade, the city has seen a bewildering series of proposed transit plans come and go: light-rail, subway, back to light-rail again, and on and on ad infinitum.

Are critics of the subway honestly suggesting Toronto should reverse itself yet again? That it should go back to Ottawa and Queen's Park and tell them the project they endorsed and funded is now off? That it should instruct transit officials to discard millions of dollars worth of planning work for the subway and go back to preparing for light rail? That it should tell the residents of Scarborough that the subway they were promised in federal, provincial and municipal elections is off the table? The last thing Toronto needs is another transit flip-flop.

The project's opponents talk as if it were some madcap scheme scribbled on the back of a politician's napkin. In fact, transit planners have been considering a subway to Scarborough, where one-quarter of Torontonians live, since as long ago as 1985. A plan called Network 2011 envisioned a line spanning the top of the city and terminating in the Scarborough town centre, the same place the new Scarborough subway would end.

The $3.35-billion cost of the project is high, yes, and in the way of such things, it keeps going up. But its opponents gloss over the fact that the alternative – replacing the outmoded light-rail line to the town centre – would come at vast expense, too. According to a rough Toronto Transit Commission estimate (disputed like just about everything else on this issue), it could cost $3-billion, not much less than the subway.

If Toronto is going to spend that kind of money, why spend it on another light-rail line along the same route, the transit equivalent of reshingling the roof? Why not go up a step and build something better and more lasting? Why not offer commuters a fast, one-seat ride downtown that saves them the time-consuming transfer from light rail to subway?

Developers told Tuesday's meeting that with a subway link, the Scarborough town centre could evolve into a busy city hub. The subway itself will be busy, too. Despite a new, lower estimate for the number of new riders it would attract – Mr. Tory's SmartTrack rail project has complicated the projections – ridership on the new line promises to be robust. The town centre station is expected to have about 30,000 boardings a day, making it the third busiest station on the Bloor-Danforth line. The station will be near highway 401, so drivers and bus riders could stop there then ride into the city by subway.

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Mr. Tory argues that in a few decades time, when the city is vastly bigger and more built up, no one will be wondering why the city extended a subway line to serve Scarborough. What they might be wondering instead is why city leaders spent so much time arguing instead of building.