I take Acela from NYP to WAS about weekly. I have found a range of service levels depending upon the crew who is assigned. I am lucky, however, that the trains I take most often are staffed by outstanding crew with excellent service. I usually tip $2-3 for good service and $5 if they have gone particularly out of their way to make my journey a good one. I have had a couple crews who also blatantly troll for tips; luckily they are usually the ones providing less than stellar service and I do not feel any remorse in not leaving them anything extra. Like any tip, however, I believe that you should leave what you feel is deserved.