Outgoing Toronto Transit Commission (TTC) chief executive officer Andy Byford says that plans to build a subway extension in Scarborough need another look if the cost, which has risen repeatedly, jumps by about $200-million more.

The province had offered to fund a light-rail line (LRT) into Scarborough. In the previous term, council decided they wanted a subway instead and that the city was willing to pay for any cost inflation.

To pay for the project, city council has imposed a city-wide levy and allocated $3.56-billion.

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The current estimate for the subway proposal has risen to $3.35-billion and could climb again next year, when a more rigorous assessment of the cost should be ready, a possibility that prompted Mr. Byford's concern.

"Beyond that [$3.56-billion] line, I think, that definitely should then re-open the debate about should there be a time-out, should we look again at the alternative, which would be an LRT," Mr. Byford said Wednesday morning.

The TTC CEO has always said that either subway or LRT could meet the transit demand in Scarborough.

In recent months, he has taken to saying the subway proposal will need to be revisited if the cost "balloons."

He had declined to specify the threshold at which he thinks this would be necessary, but offered clarity on Wednesday.

"If that cost has gone beyond the [$3.56-billion] envelope that I think we can justify, which is the figure that's in the budget … if I was still here, I would be revisiting my position," Mr. Byford said.

"That is the approved budget, so we have no authority, we would hsave no authority to go beyond that.

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"So, A, we would want to revisit that to see if that still makes sense and, B, I'm sure council would want to revisit it, because where's the extra money coming from?"

Mayor John Tory, a staunch supporter of the subway proposal, would not specify a cost at which his backing would waver.

"I don't want to draw a line with respect to the Scarborough [project], in terms of what number is the sort of number beyond which you'd say you want to change course," he said.

"I would just say I'm very committed to that project. I'm committed and I spend lots of time in meetings … finding ways to take that price down, which I still believe is possible."

The cost of extending the Bloor-Danforth subway line farther in Scarborough has fluctuated. It dropped when the project was reduced from three stations to a single one, at Scarborough Town Centre, but has been climbing steadily since.

Councillor Josh Matlow, one of the chief critics of building this subway extension, sought council support on Wednesday in his bid to ask the Auditor-General to assess the value of the project.

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The idea was voted down 27-13, with one councillor arguing it would set "a dangerous precedent."

Also on Wednesday, former chief city planner Jennifer Keesmaat weighed in on the project, noting that "there *is* a tipping point at which the cost-benefit analysis becomes questionable," in a comment posted on Twitter.

"Responsible government would continually, honestly assess this question and have alternatives in hand," she wrote.

Ms. Keesmaat resigned in the late summer. Mr. Byford is leaving this month and will take up a job in New York early next year.