机车重联，也称多机重联在铁路术语中，是指以两台或以上的机车牵引的列车。在列车需要行走爬坡路段、载重大或编排更多车辆等情况下，便需要使用「重联」。

进行重联的机车需具备性能相近的条件，假若某辆机车的速度与其他不一，其车辆会与路轨滑动并摩擦，并有机会引起多种不良后果。 一列火车以两辆或以上的蒸汽机车牵引运行，每辆机车均需司机及司炉操作。直至日后的柴油及电力机车出现，方能以一端驾驶室，透过机车间的重联插座传递指令，直接控制多台重联机车。

In railroad terminology, double heading indicates the use of two locomotives at the front of a train, each operated individually by its own crew. The practice of triple-heading involves the use of three locomotives.

Double heading is most common with steam locomotives, but is also practised with diesel locomotives. It is not strictly the same practice as two or more diesel or electric locomotives working 'in multiple' (or 'multiple-working'), where both (or all) locomotives are controlled by a single driver in the cab of the leading locomotive.