Toronto's transit agency is raising new alarms about its much-delayed streetcar order, expressing doubts that Bombardier can keep to its latest commitments.

The Toronto Transit Commission ordered 204 new streetcars and was supposed to have about three dozen of them operating by the end of last year. Instead they had three, amid production and labour problems that slowed delivery and prompted the transit agency to reject some vehicles as being subpar.

A new report released in advance of next week's TTC board meeting reveals how the agency and Bombardier worked for months to find common ground on how much production could be sped up. They reached a tentative agreement at the beginning of June – but the TTC is raising doubts only two weeks later.

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"TTC staff believe that there is a high risk that Bombardier may not be able to meet its revised schedule submitted on June 1, 2015 for year end 2015," the report reads, "and a medium risk that Bombardier may not meet the revised schedule for contract completion of 204 new streetcars by 2019."

Meeting the overall deadline of 2019 has been repeatedly cited by TTC chief Andy Byford as being absolutely non-negotiable. On Tuesday he confirmed he would accept no movement on the final deadline.

"To even entertain a slippage I think would encourage the wrong outcome," he said. "It's letting them off the hook if we say 'oh okay then, make your best efforts and we'll discuss an end-date later.' No, I think customers have waited long enough for these new streetcars."

The transit chief executive officer said that Bombardier has overhauled their workforce and production process at their Thunder Bay plant, as well as improving quality at their facility in Mexico. He has been told that Bombardier will be able to produce four streetcars a month, starting in July.

The contract is confidential but Mr. Byford said that there is "a liquidated damages regime" that is being worked through as a result of the production delays so far. It could ultimately result in what are effectively refunds to the TTC for part of the streetcars' price.

Mr. Byford is planning a trip next week to Bombardier's production facility in Northern Ontario. He is to be accompanied by councillor and TTC chair Josh Colle.

"[We] will be going to Thunder Bay to see the Bombardier plant and also speak with Bombardier officials and Bombardier workers, to obviously stress the urgency from our end … but also to see with our own eyes what the status [of the vehicles is]," Mr. Colle said at a TTC event Monday.

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"We've tried, obviously, everything within our legal toolbox to impress upon them how important this is. I think it sends a strong message to Bombardier as to how serious we are when we show up on their doorstep."

The latest delivery schedule was the result of a protracted back-and-forth between Bombardier and the TTC. According to the transit agency, the manufacturer proposed four different schedules – in January, February, March and April – before the two parties settled on one this month. The schedule has been tentatively accepted by the TTC.

But the TTC remains unconvinced it will be met, an uncertainty the report blames on documentation provided by Bombardier and the TTC's assessment of "production readiness" at the plants in Ontario and Mexico.