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Construction on the new rail link to Pearson Airport will begin in the spring. (Canadian Press )

Ontario's Liberal government won't say how much it will cost to start the long-promised rail link between Pearson International Airport and Toronto's Union Station with diesel locomotives before switching to electric trains.

Local residents in areas where the train will travel say diesel is old, dirty technology that will add to air pollution and make people sick, and want the 23.3- kilometre link to be all-electric from the start.

However, the government wants the rail link operating in time for the 2015 Pan Am Games, and that means starting the new Pearson-to-Union line with diesel trains, said Premier Dalton McGuinty.

The premier said the province was "taking advantage of the latest diesel technology," but he declined to answer repeated questions about the additional cost of starting with one type of train and then switching to another.

"Certainly what we're about to do represents tremendous progress given the existing alternatives," McGuinty told reporters Monday at Pearson Airport.

"It's a new option, it's a clean option, but we think we can in fact — over the course of time and in an affordable way — move to an even cleaner option."

New Democrat Jonah Schein represents Toronto-Davenport, a riding that will be bisected by the new airport rail link. He said the area's 300,000 residents don't want diesel trains belching additional pollution through their neighbourhood.

"People in my community of Davenport get a diesel train that is going to make people sick," said Schein.

"There's child care centres, there's seniors centres there. They don't want it, and they don't get served by it either."

There won't be any local stops in Davenport as the trains head between the airport and Union Station every 15 minutes. Each one-way trip is expected to take about 25 minutes with fares slated to be set in 2014.

When pressed on the additional costs of starting with diesel, McGuinty would only say he was confident changing to electric would be affordable.

"We'll do it over time," he said.

"We've also done it in a way, so they tell me, the track and the cars are all adaptable for an electric locomotive."

The premier should tell taxpayers exactly how much more it's going to cost to start the airport link with diesel trains before switching to electric locomotives, said Schein.

"I heard him not answer the question about how much it's going to cost," said Schein.

"I think we need to be prudent and do it properly. I don't see the point in spending money twice."

McGuinty was at the airport to announce $128.6 million in contracts for a three-kilometre spur line near the airport and a new passenger station had been signed and construction would begin in the spring.

The connector line will link the Georgetown GO line to the airport.

Last year, the entire new rail link between Union and Pearson was given an estimated price tag of $300 million. The government wasn't providing an updated figure Monday, despite the fact the short spur line would eat up more than one-third of the original cost estimate.

Experts predict the air-rail link would eliminate more than 1.5 million car trips to the airport in the first full year of operation.