Willem, I am like a dog that chases whatever squirrel is nearest, so sometimes I stray from the primary thread. How much taxable income Amtrak adds to a server's wages is too esoteric for me, trying to tip well is more interesting.
Tipping is a "gut" thing. It isn't like there is a way to precisely quantify the spectrum of tipping percentages, but it is usually better to round up, than down. I will rarely stiff a server. I start at 10% for bad service with some server attempts to remedy the negatives. 20% is my baseline for decent service. I tip more than that if the server goes over and above the normal level of service. I get back a lot more in friendly service and freebies than the amount of tips I give at places where I am a regular. The thing I try to remember is that the difference between an ok tip and a fairly good tip (15% vs. 20%) is just $2.50 more on a $50 meal.
Tipping an Amtrak Dining Car attendant if you are a sleeper customer is a grey area, I will admit. I can see the argument that 10% of the menu price is enough because of the relatively high salary that a Dining Car attendant gets vs. a typical diner waitress. So I am hesitant to come out and just say, "You should tip at least 15% in the Dining Car." And my opinion is worth no more than anyone else's. But I think that tipping a few dollars more than 10% makes little difference to me and it makes the attendants day a bit more pleasant. If it encourages good attendants to keep their jobs as attendants, then it is worth it. YMMV.
Where do you draw the lines between a good tip, a so-so tip, a tip that says "I appreciate your effort even though you have room for improvement", and a tip that says "I'm only tipping your lousy service so that you know that I didn't forget to tip"? If you can fit "You did a fine job but were hampered by circumstances beyond your control" in there, I'd appreciate it.
