noun
A train that operates during the night, often with sleeping accommodations.
夜行列車で東京から大阪まで移動した。
I traveled from Tokyo to Osaka by night train.
寝台列車 specifically refers to a train with sleeping berths, while 夜行列車 is a broader term for any train running at night.
Compound of 夜行 (やこう, 'night travel') + 列車 (れっしゃ, 'train').