Metrolinx is going back to court over a controversial $770-million light rail vehicle deal with Bombardier Transportation, questioning a key part of the judge's decision that preserved the contract.

The regional transit agency has quietly filed a notice of appeal, the latest legal salvo in a months-long spat that has soured relations between Metrolinx and their vehicle supplier. Bombardier has been winning so far, but earlier this month the province surprised the company by announcing that it had bought insurance by inking a side-deal with Alstom, a rival supplier.

The fight is over vehicles for the Eglinton Crosstown, the $5.4-billion project being built through midtown Toronto. The line, the biggest transit project under way in the country, is scheduled to open in 2021. Since last year, though, Metrolinx has sounded increasingly loud alarms over Bombardier's ability to deliver vehicles in time.

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The regional transit agency lost a legal round in its fight with Bombardier last month, when an Ontario judge said they couldn't act unilaterally to deem the company in default and walk away from the contract. It was a blow to Metrolinx, which had warned that delays could imperil their ability to open the Crosstown and other projects when promised.

The agency had until last Friday to file a notice of appeal. On Wednesday, a Metrolinx official confirmed that they had done so. The crux of the appeal is seeking clarification over the amount of time Bombardier has to fix – "cure," in the language of the contract – problems with its vehicles.

"Metrolinx is preserving our right to seek clarification of the decision principally as it relates to the permitted cure periods," agency spokesperson Anne Marie Aikins said in a statement.

The timing of this cure period is relevant because the judge's ruling meant that the two sides had to embark on a lengthy dispute resolution process, which Metrolinx officials have said could take eight to 12 months. Its latest legal gambit is an attempt to make sure that Bombardier doesn't have until the end of this period to fix any problems in its vehicles.

"We are appealing to seek confirmation that Bombardier's right to cure the defaults Metrolinx cited in its July 2016 Notice of Default does not extend to the end of the Dispute Resolution Process and beyond, but remains as agreed to in the contract (7 days to cure or provide an acceptable cure schedule)," Ms. Aikins wrote.

A spokesman for Bombardier said the firm was "disappointed" that the transit agency wanted to go back to court.

"Metrolinx is only creating further delays in this project," Marc-Andre Lefebvre said in a statement Wednesday evening.

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"Instead of playing costly legal games, we strongly suggest that Metrolinx should move the project forward and get to work on approving [the prototype] vehicles."

Metrolinx retains the right to cancel the deal at any time "for convenience," as it is called in the contract. But doing so would require that they pay a penalty to Bombardier, an amount one insider said was in "the high tens" of millions of dollars.