Loading gauge
A loading gauge is the maximum cross-sectional envelope - height and width -that a rail vehicle and its load may occupy, ensuring safe passage through tunnels, under bridges and past lineside structures.
The loading gauge is distinct from track gauge (the distance between the rails) and from the structure gauge, which defines how close infrastructure may encroach on the track; the difference between the two is the clearance.
It is a hard interoperability constraint: a wagon or trainset built to a larger profile physically cannot enter a network with a smaller one, regardless of whether electrification, signalling and track gauge are compatible. This is why loading gauge appears in vehicle authorisation, in the Register of Infrastructure (RINF), and in every routing decision for oversized intermodal loads.
In continental Europe, the reference profiles are the UIC-derived gauges GA, GB and GC, carried over into the EU’s Technical Specifications for Interoperability. GC is the largest of the three and offers the headroom needed for double-deck passenger stock and high-cube intermodal traffic, which is why it is the target profile for new-built lines.
For freight, the commercial stakes are direct: whether a corridor can carry 4 m-high semi-trailers or high-cube containers on standard wagons is a loading gauge question, and gauge enhancement is a recurring investment category on European freight corridors.