If you have a sleeper room, the car number & room number will be on your e-ticket. The car number is displayed in a small lighted window next to the door (not to be confused with the physical car number near the ends of the car). Any Amtrak employee on the platform or at a car door will happily direct you to the right car.
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If you buy a coach ticket, seats are not assigned, except for business class on some trains. An Amtrak employee will direct you to a coach car, and the employee at the car door may direct you to a specific seat or tell you what area of the car to sit in.
When approaching a station, there will usually be an announcement telling which car doors will open, and asking deboarding passengers to move to that car. If it's nighttime when there are no announcements, the conductor or car attendant should find passengers getting off and personally notify them. There's a "seat check" ticket that the conductor places over your seat that says where you're getting off, and the conductor or car attendant will try to keep track of where passengers for each station are sitting. If you change seats, it's important to take your seat check with you, and notify the car attendant if you're getting off at night or at a small station.
If the train is very long and the platform is too short, the train may make 2 or 3 stops at the station. When this happens, usually there will be one stop for coach passengers and another for sleeper passengers.