Some tunnelling may be required to build his signature transit proposal, Toronto mayoral candidate John Tory says after rivals noted that the route conflicts with construction sites and established homes.

The viability of funding and building the so-called Smart Track plan have come under increasing scrutiny with Mr. Tory vaulting into the lead in the mayoral race. Both Rob Ford and Olivia Chow, his chief rivals, have offered pointed criticism this week.

On Tuesday, Ms. Chow held a news conference where she accused Mr. Tory of being willing to "say anything to get elected." She pointed out that parts of the right-of-way where the rail line would run, an area along Eglinton Avenue that had once been set aside for an expressway that was not built, had been sold for development.

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"Mr. Tory has to answer a very simple question to the voters of Toronto, which is this: How can he build 12 kilometres of heavy rail from Mount Dennis, which is Weston, all the way to Mississauga without tunnelling, without buying land and without having to [go on] the road," she said.

Last week, Mr. Tory appeared to rule out going underground for his proposal, which builds on the province's plan to electrify GO Transit rail and move to more frequent service.

"This can be done in seven years 'cause it's on existing GO train tracks – no tunnelling, no buying of property," he said at a debate organized by The Globe and Mail and the Toronto Region Board of Trade.

He did not answer when Mr. Ford later challenged him to explain what he would do west of Mount Dennis, where there are not tracks now.

In an op-ed piece on Monday, Mr. Ford accused Mr. Tory of having "a different answer each week."

"What will you do along Eglinton, in Etobicoke, where there are no existing rail lines or available right-of-way to fit your surface rail plan?" he asked in a column in the Toronto Sun.

At City Hall on Tuesday, Mr. Ford said the people in Etobicoke "definitely don't want trains going down the middle of the road."

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The John Tory campaign said the candidate's reference to not having to tunnel was not meant to apply to the whole route, only to the portions where tracks already exist.

And Mr. Tory told a television station that other parts might have to go underground. "The fact is, there may have to be some tunnelling," he told CP24. "We will use tunnelling if and when and where necessary to get Smart Track built."

The Tory campaign did not immediately respond to questions about how much tunnelling would be required and how much could be done within the $8-billion cost estimate it has offered.

Also Tuesday, the campaign released an interactive Web tool that aims to show commuters how much time they would save if Mr. Tory's transit plan goes ahead. According to the site – http://smarttracker.ca/ – the advantages start to be seen for people going downtown from Ossington Avenue and points west and from east of Greenwood Avenue.

With a report from Marcus Gee.