The term 'SLEEPER OR DINER' can be solved by the three letter word 'CAR'. A sleeper or diner CAR is a type of railroad CAR that is used for passenger service. It is designed to provide sleeping and dining accommodations for passengers on long-distance trains. The cars are usually equipped with beds, showers, and a dining area. They are also sometimes used as lounge cars, providing a place for passengers to relax and socialize during their journey.