What track gauge is used by London Underground trains on most of the network?

London Underground trains use the standard track gauge of 1,435 millimetres on most of the network.

This gauge measures the distance between the inner faces of the two rails. It became the standard for Britain’s main-line railways and was adopted by most Underground lines, helping trains connect with or share infrastructure with national rail routes.

The Underground is not completely uniform. Some lines have unusual loading gauges, clearances, platform arrangements, and electrification systems, even though standard gauge is dominant. These physical differences are why trains from one Tube line generally cannot simply run on another.

Track gauge is often confused with loading gauge. Gauge describes rail spacing, while loading gauge describes the maximum train size that can safely pass through tunnels and stations.