As has been pointed out, tips are not required. Also, as has been pointed out, Amtrak OBS are not like most service workers in that they are relatively well paid. Thus, one should not feel like you are "stiffing" anyone by not tipping. That said, if you've just paid 100s of dollars for a sleeping car ticket, you probably have enough money to afford another $20 - $30 in tips to service workers you encounter along the way. If you're taking a long train trip, you should remember to bring the extra cash, and in small bills, too. Of course, if the service is substandard, there's nothing wrong with not tipping, especially because the service workers earn a decent wage.
I personally tip OBS. I tip the SCA $5 -$10 a night, depending on the quality of the service. For traditional full-service dining, if it ever comes back, I estimate that the Dinners are around $25 -$30, so I will tip $3 -$5, which is about a 15% tip. Breakfasts and lunches are priced at about $10, so I tip $1.50 - $2.00. On the trip when I did flex dining, I didn't tip on the Capitol when I had to line up, pick up my tray and take it to my seat and then put it in the trash myself, but I did tip the attendant on the Cardinal, who served me my flex dinner as if it were a real dining car meal.