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).
下り線の電車はまもなく到着します。
The train on the outbound line will arrive shortly.
事故のため、下り線が一時運転を見合わせています。
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.