I've logged a lot of miles on Amtrak and from what I've seen, tipping is most definitely the norm, especially amongst well traveled passengers.

Even though meals are included in First Class sleeper fare, I still tip the dining car staff the same 15-20% as if I were in a restaurant. They are working just as hard. As for the sleeping car attendants, they work very hard, over long hours, with not a lot of quality sleep time. I will tip them a minimum of $5.00 per day. I've had some SCAs that just provide the basic services, but for those who go about their jobs in a friendly, outgoing manner and anticipate rather than await their guests needs, I'll usually throw in an extra $5 to $10 dollars.

For me at least, there are no concerns as to whether they are well paid or not.