A Toronto citizens group is taking Metrolinx to court over its plan to run diesel trains to Pearson airport, an upping of the ante that could prevent the new express rail service from opening in time for the 2015 Pan Am Games.

The Clean Train Coalition (CTC), a non-profit organization of west-end residents, is asking the courts to kill the diesel plan in part because the 2015 deadline was arbitrarily imposed by Metrolinx's overseers at Queen's Park, according to the CTC.

"The board [of Metrolinx] decided to proceed with diesel trains to advance the short-term goal of hosting the Pan Am Games. Metrolinx's 2015 deadline did not originate from either its board or its staff ... this instruction originated from the province," the CTC says in its application for judicial review, filed with the Ontario Superior Court of Justice Aug. 2.

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The CTC, which declined comment Tuesday, is expected to speak about the legal action at a news conference at 11 a.m. Wednesday.

The issue of the 2015 deadline is important because officials at Metrolinx -- the province's transportation planning authority for the Greater Toronto Area and Hamilton -- have said it would be impossible to electrify the Air-Rail Link in time for the Pan Am Games.

The summer sporting extravaganza has served as a catalyst for the long-awaited express line connecting Union Station and Pearson airport.

But the project's completion date could be jeopardized if the CTC succeeds in tying up Metrolinx in court over its decision to run diesel trains on the route.

Metrolinx declined to comment on the legal challenge Tuesday, but its spokeswoman said in a statement that "work is continuing on the ARL and we're on track to be up and running for 2015."

Metrolinx has already committed to spending $53-million on at least 12 clean diesel trains built by the U.S. arm of Japanese train-maker Sumitomo Corporation, with an option to buy six more.

They can be converted to electric for $1-million each.

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"The ARL will launch with Tier 4 diesel multiple units. These vehicles are fully convertible to electric, beat the World Health Organization's stringent emissions standards and reduce airborne particulate emissions by 90%," Metrolinx spokeswoman Vanessa Thomas said by e-mail.

Last year Metrolinx's board endorsed in principle a plan to spend $1.8-billion over 20 years electrifying the GO system's busiest lines. The ARL, which alone would cost $457-million to electrify, is supposed to be first.

So far the cash-strapped McGuinty government hasn't committed any funding beyond $15-million to conduct an environmental assessment of electrifying the ARL, a study that's supposed to be finished in 2014.

The CTC has been a thorn in Metrolinx's side for the last few years, drumming up a vocal and tightly organized campaign against more diesel trains chugging along GO's Georgetown corridor, which passes through a mix of poor and gentrifying residential neighbourhoods including the Junction, Roncesvalles, High Park, Weston and Mount Dennis.

Trains serving the new ARL express line would run from downtown along the Georgetown South corridor before splitting off on a spur to Pearson. More than 300,000 people live within 450 metres of the line, according to the CTC.

The CTC argues in the court documents that Metrolinx didn't do a proper comparison of diesel and electric vehicles, and that it should reconsider its conclusion that the adverse health impacts of more diesel trains would be minimal in light of the World Health Organization's recent reclassification of diesel exhaust as a human carcinogen and cause of lung cancer.

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"The decision to invest in DMUs [Diesel Multiple Units] was not based on a 'triple bottom-line' evaluation of the relative environmental, economic and social impacts of the reasonable alternatives for this project," the CTC says in its application for judicial review.