Sorcha, I so agree!
To the OP's question, while not required, in practice in the US tipping is customary in restaurants, to cab drivers, those who help with luggage, etc., and is expected unless your service was terrible. In sit-down restaurants, most people tip around 15% to 20% if service is reasonably good. 10% is considered on the low end for a tip in a full service restaurant.
When we traveled on Amtrak, we tipped in the dining car roughly based on the menu prices although our meals were included in the price of our sleeper. We also tipped the sleeping car attendant even though our travel (daylight hours only) did not require use of the bed. I don't remember how much, probably $5 to $10 depending on how often he or she checked in on us, helped with bags, etc. Probably even more the few times we had meals brought to our compartment.
Without considering any debate on whether this is fair/reasonable/logical, in the US it is usually considered stingy or being somewhat of a cheapskate if a tip is not given, unless the service was horrible. I consider it a way of thanking the worker for his or her attention and service. Not saying you must, of course, but just to make you aware of what is customary in this country. Best wishes for a great trip!