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.