In the late 1990s, the King-Spadina area of Toronto had 945 residents. My friend Brett Grainger was one of them: He lived with a roommate at 525 King St. W., above a pub called The Toad in the Hole. The block was "central, yet so hidden away, almost sleepy," he recalls. "The neighbourhood vibe was seedy and urban, in a grungy, Montreal kind of way, and I loved it."

Nobody was imagining a "Rail Deck Park." Now, everything has changed.

King and Spadina is neither sleepy nor seedy. About 19,000 people now live in the one-square-kilometre area, a number that the city estimates could grow to 40,000. Brett's old building has been gutted and rebuilt; the pub is now a Starbucks. One of the best chocolatiers in Canada is nearby. The sidewalks, which were always empty, are jammed with lunching Web developers and mothers on mat leave. A dozen new condo buildings have gone up within a block, and more are coming: If one developer gets its way, 525 King will be engulfed by a mountain-shaped stack of 630 apartment units.

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Rail Deck Park, if it is built, will be a few steps away. This week, City Council voted unanimously to study the proposal for a 21-acre park above the rail corridor between Bathurst Street and Blue Jays Way, right in this neighbourhood. It would be expensive. It would be complicated. It would be big.

But not big enough. A 21-acre park, by itself, won't match the galloping growth of this area, which is absorbing thousands of new housing units that Toronto's established neighbourhoods are fighting to keep out and pumping up the city's coffers in the process.

This is why the planning department is thinking big. It has a much broader Parks and Public Realm Plan for the downtown, part of TOcore – a still-larger effort to rethink the policies that are guiding growth.

Imagine a grand "loop" of parks that connects the Don Valley with the Port Lands, the Toronto Islands, Fort York, the new park under the Gardiner Expressway and more. Then imagine smaller parks and renewed streets that open up ravines; improved streets and new uses for parkettes; events and schemes that open up libraries and churches and schoolyards.

This vision – developed with consultants, including smart landscape architects Public Work – is nimble and sophisticated, not just green squares on the map, but also a set of tactics for an unpredictably growing city.

"We have to be opportunistic and innovative," says Ann-Marie Nasr, manager of strategic initiatives at the planning department.

"Everyone feels the economic effect of our downtown growing," Ms. Nasr adds. But, with parks and other infrastructure, "we're playing catch-up," she says. "The growth has happened at a rate that was unexpected, and it's kept happening."

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Ms. Nasr illustrates with a graph: The line marking downtown Toronto's population is more or less flat until the late 1990s, and then starts turning steeply upward. And then it keeps going and going. Planners predict that it will almost double to 475,000 by 2041.

If the downtown doesn't get the parks and public amenities it needs, it will be a grim place to live. And trying to engineer parks into a developed city neighbourhood is pricey and complex, a problem that King-Spadina already faces.

This is what the Parks and Public Realm Plan is designed to address. It includes a study of how people use downtown public spaces, and a comprehensive rethink that will guide planning policy in advance.

Rail Deck Park is just one piece. Yet it is a crucial one. "The biggest challenge is to achieve a very large park," city planner Sarah Phipps says. That's because "the type of growth we're seeing now, infill on much smaller sites, doesn't allow us to get that through development charges," she says.

Ms. Phipps is referring to the cash fees that developers pay toward new parkland when their sites are small – which, in downtown, is almost every single time. The city is reviewing the details of that policy, which right now allows the developers of very large buildings to pay a lesser proportionate share.

Money, of course, is a big question. The idea of another capital project worth $1-billion or more, on top of the city's roughly $30-billion in unfunded "priorities," seems to many either preposterous or obscene.

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Joe Cressy, the ward councillor who is pushing the Rail Deck vision along with Mayor John Tory, is unapologetic. "It's about ensuring the livability of the city, and about creating signature public spaces," he says. "There's no question that will cost a significant amount of money – and that's worth investing in."

This is true, in both quality-of-life and fiscal terms. A "signature" park of this scale has the capacity to significantly boost the city's international image, real-estate prices, attractiveness to employers and immigrants – as Mr. Tory has been arguing.

It's not at all clear whether the mayor is ready to actually raise the money to fund this grand vision. While plumping for the park plan, he also continues to demand more austerity budgets: City departments, including parks, are being asked to make 2.6-per-cent budget cuts at a time when costs, population and the demand for services are all going up.

This is ludicrous. Many Toronto public spaces, as Globe and Mail urban affairs columnist Marcus Gee wrote this summer, are embarrassingly ragged already.

Big-ticket projects are hard to sell in post-Rob Ford Toronto (unless they involve expressways). A decade from now, however, that could be different – especially with tens of thousands more downtowners cycling, getting stuck on transit and trying to find somewhere to take their kids to play soccer.

Things change. In 1998 and 1999, Brett Grainger went out each day, "walking [my roommate's] chocolate lab past the strippers taking a smoke break outside For Your Eyes Only," he recalls, "and over to that crappy parkette." Victoria Memorial Square, that is.

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I went there this week. It has been rebuilt and looks great: Dog walkers share the green space with restored colonial headstones. Seven buildings, with more than 1,000 apartment units, overlook it. And the new playground was packed.

A big new park seemed like a good idea.