down train; outbound service
Refers to a train, bus, or flight traveling away from Tokyo or a major hub toward a more remote destination. The opposite is 上り便 (のぼりびん).
次の下り便は何時に出ますか。
What time does the next down train leave?
下り便のバスは一時間に一本しかない。
There is only one outbound bus per hour.
Compound of 下り (くだり, 'downward') and 便 (びん, 'service, flight, mail'). The directional sense follows the traditional Japanese railway convention where trains toward the capital are 上り and those away are 下り.