The decision on the future of Scarborough's transit is being bounced back to Toronto city council, with the head of Metrolinx saying that any changes to the original LRT plan will require a council vote to reopen the current agreement.

Metrolinx Chair Robert Prichard released a public letter Tuesday in response to Toronto Transit Commission Chair Karen Stintz's written concerns about the province's plan to build a shortened subway route in Scarborough instead of a light-rail line, or the longer subway line endorsed by city council.

After the Metrolinx Board of Directors meeting on Tuesday morning, Mr. Prichard defended Metrolinx's decision to move forward in discussions of a shortened subway and evaluations of the Scarborough Rail Transit corridor. Along with the written response to Ms. Stintz, Metrolinx also released a feasibility study on the new subway route that would run along the same path that the Scarborough Rapid Transit line runs on now and end at the Scarborough Town Centre .

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"Using the existing SRT alignment, which is also the approved alignment for the LRT in the master agreement, is worthy of the most serious consideration," Mr. Prichard wrote in his letter. "It offers the possibility of a better solution at lower cost serving more people and generating greater economic growth than any other alignment."

Last week, Ontario's Transportation Minister Glen Murray announced the province would contribute $1.4-billion to a shortened subway route in Scarborough, contradicting the binding master agreement that specifies light-rail transit plans. The master agreement, which is signed by the city, TTC and Metrolinx cannot be amended without City Council's vote.

City Council could decide on whether or not it wants to re-open the master agreement as early as October, Mr. Prichard said.

Although he wrote in his letter that a subway from Kennedy to Scarborough Town Centre would be delivered close to the existing provincial funding commitment, the city will still face some heavy costs. If the LRT plan was to be abandoned by council, it would cost the city roughly $85-million, plus the cost of re-negotiating. Funds for shuttle buses and overrun costs would also be borne by the city, according to Metrolinx.

"Any risk associated with the delivery of the project is held by the owner of that project," said President and Chief Executive Officer of Metrolinx Bruce McCuaig.

Other issues outlined in the feasibility report include major redesign costs and the stoppage of service on the SRT for up to three years.

Andy Byford, CEO of TTC, said the new SRT proposal looks suitable at first glance, but there are many details that have yet to be discussed.

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"We haven't accepted that the route is technically viable," Mr. Byford said Tuesday. "There are still plenty of engineering challenges ahead, such as inclines and curves."

He said it would be further discussed at a TTC board meeting Sept. 25.

With reports from Elizabeth Church and Oliver Moore