It remains customary to tip any Amtrak on-board service (not operating) staff for good service received. The most widely discussed aspect is how to tip sleeping car attendants. A popular practice is to start at a baseline of $5 per passenger per night, moving up or down depending on service. I always tip $5 as soon as I board the train to curry favor with the attendant. If they're subsequently invisible, no more tips.

In Acela FC, I start at a baseline of $5 for the entire trip, leaving a few singles after the first drink service.

In other short-distance Business Class (not Acela) where there are attendants, I may or may not tip at all depending on their visibility.

YMMV.