Roomette. For customers seeking both privacy and savings in First Class, Roomettes feature two comfortable seats by day transformed to upper and lower beds by night. Each room includes a big picture window, newly upgraded bedding, pillows, towels and linens and access to a restroom and shower in your car.
A dining car (American English) or a restaurant car (British English), also a diner, is a railroad passenger car that serves meals in the manner of a full-service, sit-down restaurant.
Compartment and cabin would both work. The difference between the two words is that a cabin is meant for people, while a compartment could be for people, or for other things like baggage.