Tipping even at restaurants is confusing these days. But on Amtrak, for me it is simple--10 dollars per person per night in a sleeper to the SCA, less is service was perfunctory, more if above-and beyond; and if surly or begruding, nada--like when our SCA on the Empire Builder informed me that if we wanted our beds made up, he'd do it at 6 pm--otherwise we'd have to fend for ourselves. I do a standard 3 dollar per person breakfast tip, 5 per person for lunch and 10 per person for dinner, again with more for above-and-beyond service, and none for rude service. As has been emphasized here, tipping is optional, but this is my preference.
Biggest problem is having enough actual cash on hand--I better get to the bank while I'm thinking about our upcoming trip and stock up now!