sleeper train; sleeping car train
Refers to a train with sleeping berths, typically for long-distance overnight travel.
I traveled from Tokyo to Sapporo on a sleeper train.
寝台車 refers to a sleeping car (a single carriage), while 寝台列車 refers to the entire train service composed of such cars.
Compound of 寝台 (shindai, 'bed' or 'sleeping berth') and 列車 (ressha, 'train').