Plans for downtown Toronto's first subway in decades are taking shape, with the city's planning department urging that it run below Queen Street.

Details of the long-awaited downtown relief line – a route that has been discussed in various permutations for a century – emerged on Friday. According to information obtained by The Globe and Mail, staff have concluded that the best approach involves a connection from Pape Station near Danforth Avenue to the area around City Hall.

Although the plan is primarily about diverting passengers from the overcrowded Yonge subway line, a briefing for councillors made clear the value of the new line to the city centre as well. According to a draft staff presentation, the subway plan would "fill [a] rapid transit void in the core" and "recognizes that downtown is 24/7."

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The proposal pencils in stations along Queen Street around Sherbourne Street, Sumach Street and Broadview Avenue, and one near Gerrard Square. These would allow access to Regent Park and Moss Park, and offer the chance of a connection to the Stouffville GO corridor, which is expected to get much more frequent service under provincial and city plans.

Ridership projections for the proposed line are expected in the next few weeks, and the plan itself will form part of a broader package of transit proposals going to city council in June. Future extensions would push the line farther north and west. But no funding for any of it has been secured, and construction of even the first phase would likely take at least a decade.

Surging ridership on the Yonge subway line in recent years has made clear the importance of a downtown relief line. Officials including Toronto Transit Commission CEO Andy Byford and the city's chief planner, Jennifer Keesmaat, have repeatedly described the relief line as the city's top transit priority. Mayor John Tory, who ran on a more immediate plan to relieve the subway by adding service on local GO rail corridors, has said the DRL would still be needed in the longer term.

"We have to take action now," said Councillor Josh Matlow, one of the stronger voices at city hall arguing for the DRL.

"As Toronto's finally moving on expanding our rapid transit system … based on growth projections, the relief line addresses an existing condition that's getting worse every year. Many of our streetcar lines, along with the Yonge [subway] line, are already overcrowded."

The preferred alignment that emerged on Friday – which must still go through a public consultation process and be formally recommended to council – would avoid adding passengers to the increasingly crowded Union Station. By moving farther north than earlier proposed routes, it also would reduce potential conflict with plans for more service along the GO lines. And it would offer an east-west alternative through the downtown.

"We are very happy to finally see some concrete work being done on this proposal," said Louis Mark, a founder of the Toronto Relief Line Alliance, which plans to start ramping up its advocacy. "We have to keep working to make sure as many people as possible know about this proposal, and about its benefits, to make sure it has as much support as possible."

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City planning staff looked at six possible routes. These all started from Broadview or Pape stations and made their way by various routes downtown. The alternative laid out on Friday got top marks in the most categories, including affordability, public health and environment, choice and experience.

One key advantage the route planners say, is that it would create a station by Nathan Phillips Square in "the geographic and psychological centre of the city." This route also has what staff described as the lowest cost of the options, by providing a shorter crossing of the Don River and avoiding pricey soil stabilization required if passing under the river farther to the south.