I'm an experienced passenger, here's my sleeper attendant tipping philosophy:
I don't tip our sleeper attendant unless they give good service, are open to hearing of our needs, and avoid the too-common underlying surliness. They must be kind and willing to take care of our simple needs without complaint, and not treat us like they were parenting. And they must adhere to the employee handbook with regard to passenger relations.
It's tough on many Amtrak employees these days because of under-staffing. But sleeper attendants are different, on my routes they still are one person to a car -- same as always.
They don't rely on tips, they do well without them. They are not like the typical below minimum wage hotel or restaurant worker. Sure, there are long hours and days at a time away from home, and that inconvenience is a factor, but they are fairly compensated for that.
Their the average annual salary is just under $55,000 per year. And if the average tip is $5 a night, with 20 rooms that's another $90 a day avg. x 5 days avg. x 50 weeks avg. = $22,500 per year.
(That said, give us good service and a good attitude, I'll usually tip $20 - $25 for our 2-night trips. Because I want that level of service again, and I want it for others too.)