I have no idea. I haven't worked since 1989.
But at the time I was working, if I was called for a trip and I didn't want to make it because I felt unfit, it was basically no big deal. I worked a lot. I always worked when they wanted me and I felt that I was fit, which was most of the time. The scheduling procedure is guesswork: you're expecting to go to work at a certain time, and it turns out you're going in much earlier or much later; you don't get enough sleep, and so on. Whenever I booked off a trip because I simply didn't feel fit to make the trip, it was no problem.
What I've heard from some people I know who are still working for CN is that now harsh discipline is imposed for refusing a trip when your turn comes up. If that's the case, that in itself would be a serious issue regarding safety--essentially forcing employees to go to work when they don't feel fit to do so.