[Zircom]

To my understanding "being tired" (which is absolutely part of being hungover the next day after a night of heavy drinking) is comparable and according to some sources pretty much equivalent to being "under the influence" in terms of how much it can impair you when operating heavy machinery.

[antisyzygy]

I know a couple railroad engineers. They basically had to give up drinking because they could lose their job.

They do some kind of test that can pick up if you drank in the last couple days.

The way they assign shifts too causes the problems. If a shift pops up, they wanna take it, and that could happen at any time.

In any case I'd wager it's related to the hangover problem. You don't want a tired, distracted (by headaches or whatever), grumpy person moving 12000 tons.