sidetrack; siding
Railway term for a short track connected to the main line, used for storing or passing trains.
貨物列車が側線に入った。
The freight train entered the siding.
lateral line (of a fish)
Biological term for the sensory organ along the side of fish and some amphibians.
魚の側線は水流の変化を感じ取る。
A fish's lateral line detects changes in water flow.