侧线（Siding），在铁路工程是指从道岔引出的侧向铁道，一般与正线相对。侧线是此类线路的统称，包括到发线、越行线、机走线等。

A siding, in rail terminology, is a low-speed track section distinct from a running line or through route such as a main line or branch line or spur. It may connect to through track or to other sidings at either end. Sidings often have lighter rails, meant for lower speed or less heavy traffic, and few, if any, signals. Sidings connected at both ends to a running line are commonly known as loops; otherwise they are known as single-ended sidings or dead end sidings, or (if short) stubs.