down line; outbound line
Refers to a railway line heading away from a central terminal, typically Tokyo. The opposite is 上り線 (のぼりせん).
The train on the outbound line will arrive shortly.
Due to an accident, the down line is temporarily suspended.
Compound of 下り (くだり, 'going down') and 線 (せん, 'line'). The 'down' direction is historically based on movement away from the capital.