The TTC wants to hire an outside engineering firm to finish the overdue, over-budget Spadina subway extension at a cost of $150 million over the project's $2.63 billion budget.

But it will be the end of the year before it's known how much over-budget the project really is. That is when TTC CEO Andy Byford said he will report the cost of legal claims between the TTC and its contractors.

He refused to speculate on the dollar figures attached to those claims for the problem-plagued six-stop subway from Downsview to Vaughan, saying only that, "The figure is large."

Toronto Star sources have said the project is about $400 million over budget.

Claims over job changes, soil conditions and delays in getting one contractor to finish a piece of the job so that another can begin, are standard on big builds and not unique to the TTC, said Byford.

A report to the Toronto Transit Commission which governs the TTC, released Friday, suggests the TTC may be ready to step back from its traditional role of managing transit construction. It recommends sole-sourcing the project management as the cheapest of four options and the one that would provide the earliest opening of the line -- the end of 2017 -- two years after the originally scheduled 2015 launch.

"I think we should look at other (project delivery) models going forward," said Byford, adding, however, that the TTC has already been directed to build the Scarborough subway.

The most expensive option outlined in Friday's report, leaving the TTC to oversee the remaining 30 per cent of Spadina, would cost $185 million and result in a mid-2019 launch.

9 projects gone wrong

The TTC is not recommending a partial opening of the line as far as York University, the key destination on the extension. It would cost an additional $12 million and only speed up the opening of the first stations by two to three months, while delaying the opening of stations north of the university by six months, said Byford.

"The more you delay, the more project management costs you attract and the greater the risk of further claims," he said.

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The city's portion of the $150 million outsourcing would be $90 million with York Region on the hook for $60 million. None of that would be needed immediately since there is still about $1 billion of the original funding to be spent by the end of 2016.

A key role of the external project manager would be to "re-set" the "deteriorating and strained relationships" between TTC staff and its contractors, said Byford. That strain was "a driving force", he said, behind his decision to fire two senior project executives on Thursday.

The subway was behind schedule almost from the beginning with a funding delay stalling work for 18 months. The schedule was never adjusted accordingly. There were also issues with power and sewer lines, the foundation of the Schulich building at York University and a four-month shutdown following the death of a construction worker.

With files from Noor Javed