Mayor Rob Ford has opened the door to supporting a downtown relief subway line, saying he will leave it up to the TTC's chief executive officer to decide which of Toronto's possible new underground lines should be built first.

"It's up to Andy Byford," Mr. Ford said, adding he is "open to the idea," of the downtown relief line if it benefits the city and TTC riders.

"I campaigned on subways. We are going to get subways, the relief line, the Sheppard line, Eglinton hopefully will go underground. Subways are coming to this city one way or another."

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Although the decision ultimately lies with city council, Mr. Byford made it clear Monday that he believes the downtown relief line should be built before any others within city limits, including the Scarborough subway the mayor has championed since he took office.

Asked whether the DRL should be the TTC's first priority, Mr. Byford said: "I'd say so, yes."

The trick, Mr. Byford told The Globe and Mail's editorial board Monday, will be convincing politicians and the public that the DRL would benefit transit passengers across the city, not just the urban elite in the core.

"The challenge for me is to get some people off this misconception that because of the name it's some kind of private line for people who live downtown. Well clearly it's not," he said.

The TTC's board will discuss a new report this week that warns of dire consequences if pressure is not taken off the already jam-packed Yonge-University-Spadina line and the subway system's overcrowded nexus at Bloor-Yonge station.

Ridership in the city's core is expected to rise by more than 50 per cent by 2031 – and the deluge of new riders cannot be prevented by adding larger streetcars and subway trains or improving GO rail service, the report says.

As a partial solution, the report recommends building a $3.2-billion first phase of the downtown subway relief line from Pape station on the Bloor-Danforth line to St. Andrew station at King Street and University Avenue.

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The report proposes a western extension of the line from St. Andrew to Dundas West station, increasing the total cost of a DRL to $6.2-billion. Mr. Byford pointed out that the eastern leg could be extended to meet up with the Eglinton LRT to serve even more suburban commuters looking to travel downtown.

An unfunded DRL is included in long-range plans by Metrolinx, the province's transportation agency for the Greater Toronto Area and Hamilton.

However, according to its current ranking in the Metrolinx queue, the DRL would not be built for at least 25 years.

"People often say to me, now what about extending the Yonge line north of Finch? … But I just don't see the rationale in going hell-for-leather for such an extension when all you're going to do is add yet more pressure on Yonge and Bloor," Mr. Byford said. "At the very least, we've got to be investigating these things in parallel because otherwise all we're going to do is end up with absolute gridlock."

An extension to the Spadina subway into Vaughan is already under way, but its completion date has been pushed back to the fall of 2016 from the end of 2015, according to another report going to the commission meeting Wednesday.

TTC commissioners will also be looking Wednesday at awarding a contract to make cellphone service available in subway stations, beginning with two stations as a pilot project.

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A new report, released Monday, recommends awarding the contract to Broadcast Australia, which would pay the TTC $25-million over 20 years for the rights to operate an underground wireless network for the TTC.

Despite the challenges of overcrowding on parts of the subway system, Mr. Byford has concerns about seeing those lines uploaded to the province, as Progressive Conservative Leader Tim Hudak is advocating.

Mr. Hudak released a white paper earlier this month recommending Toronto's subway and future LRT lines – along with its expressways – be uploaded to Metrolinx, leaving the TTC to run streetcars and buses.

"What does worry me is the suggestion that the jewel in the crown of the TTC, if you like – namely its multi-modal capability – would be split up. I would not be supportive of the subway being split from the buses," he said, adding that losing the profitable subway lines would put a heavy financial burden on what is left of the TTC.