noun
A railway siding or turnout track where trains can wait to let another pass or be temporarily stored.
貨物列車は待避線で特急の通過を待った。
The freight train waited on the siding for the limited express to pass.
側線 is a general term for a side track, often used for loading or storage; 待避線 specifically emphasizes waiting to let another train pass.
Compound of 待避 (たいひ, 'evacuation; taking shelter') + 線 (せん, 'line; track'). The term is used in railway contexts to refer to a track where trains can wait or be shunted aside.