Unifor National President Jerry Dias says Bombardier walked away from talks on Saturday and subsequent conversations have not brought the two sides much closer.
The union has said its 900 members will walk out at 2 p.m. ET Monday if a deal is not reached.
Dias says the company wants to make changes to pensions and benefits that are unacceptable and the union is also worried about outsourcing of work and possible future layoffs.
A Bombardier spokesman says the company is determined to reach a deal where it remains competitive and can help it to bring more work to Thunder Bay.
There was a brief strike in 2011 over similar issues and Dias says Bombardier should be under no illusions about the workers' determination.
”They know our history, they know that we’re prepared fight them on it and I don’t know how they would believe in their wildest dreams that we’re somehow going to change tack three years later,” Dias said in a phone interview Sunday night.
There was no "significant" progress between the two sides as of Sunday evening, Dias said.
"We've had a lot of discussions, he said. "But we seem to be stuck in the mud, but hopefully things will change as we get a little closer to the (strike) deadline.”
The Thunder Bay facility makes rail and mass transit cars for cities, including Toronto's subway system.