They are well aware of passengers getting on the train... and accommodate. A good attendant will accommodate meal requests when you board as they greet you.If for some reason they haven't, go to the diner and get one from the LSA.
That said... there are a very few Amtrak employees who provide service above and below that standard of basic expectation. I have had the below expectation experience many times. In that situation just go to the dining car and tell them you just got on. They must accommodate you and know you are entitled to appropriate service as per your ticket.
I'm not happy to report I have been overlooked regardless of attempts to be accommodated with paid services. Sorry to say... tip the customer supportive employees well... and don't tip those who would rather be playing computer games than make your traveling all that you paid to have it be.
Yup... I've seen it all.