Top tranist officials say they plan on sticking to the city's plan to build a network of light rail lines across Toronto as outlined in the Transit City plan. (Bombardier)

Top transit officials are throwing cold water on promises made by some of Toronto's leading mayoral candidates to rework the city's transit system by adding to the subway network.

Some mayoral candidates have criticized the city's long term transit plan, dubbed Transit City, in which Toronto would be criss-crossed with a number light rail transit lines. Transit City does not include plans for the construction of any new subway lines, which are generally far more expensive to build.

Four of the candidates — Rob Ford, Rocco Rossi, George Smitherman and Sarah Thomson — have pledged to either build new subway lines or extend and amend existing ones.

Only deputy mayor Joe Pantalone, a staunch ally of outgoing mayor David Miller, has said he wants to stick to completing the existing transit city plan.

The head of Metrolinx, the provincial agency responsible for transit in the GTA, says it's too early to give any of the proposals to enhance the city's subway network serious consideration.

"There's never been any detailed work that's been done on a majority of these [candidate proposals], so I can't comment on their deliverability. I can't comment how much time it would take [or] how much it could cost to deliver on those projects," said Bruce McCuaig, the president and CEO of Metrolinx.

He said Metrolinx is currently committed only to building components of Transit City in the coming years — namely the Sheppard, Finch West, Eglinton Crosstown and Scarborough LRT lines. The province has already allocated funding to those specific projects, he added.

"It's great to see that transit and transportation issues are such a prominent focus in the discussion in the campaign," said McCuaig.

"And we will be working with the new mayor, the new council as they deal with these issues going forward, but right now, our focus is on delivery of the projects that have been approved and have been funded."

Subways cost $250-300M per km

Gary Webster, the general manager of the TTC, said there are "significant challenges" to reworking the city's transit plan, particularly when it comes to funding and the time it would take to plan and implement the changes.

"The corridors that are under review that we are talking about don't require a subway system and you get much more bang for your buck with an LRT," he said.

"We're just very concerned about how much money is there, what's the best use of it, what's the appropriate mode given the amount of people we have to carry."

Webster said the TTC has met with two of the leading mayoral candidates to discuss the pros and cons of their transit plans. But he would not reveal who those candidates are.

According to the TTC, the estimated cost for building subway lines in the city can range from anywhere between $250 million and $300 million per kilometre. The costs to build the four LRT lines prioritized by Transit City, meanwhile, range from $63 million per kilometre to $153 million per kilometre.

Subway trains, on the other hand, can also carry more people. The average capacity of a six-car subway train is estimated at around 1,100. Meanwhile, the Bombarider light rail vehicles that will run in the city have a total capacity of around 175.

In the March budget, the Ontario government said it was postponing delivery of $4 billion to fully implement the Transit City plan.

Initially, all projects were to be completed by 2018. Since the March budget, however, Metrolinx has pushed back its timeline. The Eglinton Crosstown LRT and the Scarborough LRT, the last two lines to be completed, are now only expected to be fully ready by 2020.