If a certain group of workers is too critical to be allowed to strike, then they're too critical to be forced to work in poor conditions. "Back to work" legislation should not exist to bind the workers, but bind the companies.
"Sorry, you're too important to Canada's infrastructure, you can't be allowed to mistreat your workers. As punishment for letting it get this bad, you must agree to ALL of their terms, and in return, they'll begin working first thing tomorrow. Guess you should have been more agreeable BEFORE the strike."
I dunno, forcing strikes to end in any situation seems dangerously close to forced labour. But maybe I'm just a dirty commie.
(edited a spelling mistake)