A through train (also through service, run-through service/train) is a concept of rail transport (commuter rail, subway systems, and mass transit) that involves a change in the operating provider of the line, or a change in the identity (usually operational) of the line, at a specified boundary, on a regularly specified schedule (not random). This is usually accomplished through compatible infrastructure—identical track gauge and durability issues (although variable gauge trains do exist, they tend to be expensive), rolling stock dimensions, curve speed and signaling compatibility, train station dimensions (to avoid damage to rolling stock), tunnels and bridge dimensions and maximum weight, and power requirements. The exact terminology (and definition) vary as usage; in the case of National Rail of the UK, a through train is one which may be used by a passenger to make their entire journey without changing trains.

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