I think the issue is that the railroads are running thin crews and extremely tight schedules. So Jim the engineer calling in 3 hours before he is due to commandeer 2,500 tons of freight for 500 miles creates a dire situation that not only delays that train, but throws off the schedule of dozens of other trains.

I'm just hypothesizing here, and I feel like I am missing something since it doesn't seem like having/adding on-call employees was an option.