Structure gauge

The specified amount of clearance makes allowance for wobbling of rail vehicles at speed or the shifting of vehicles on curves; consequently, in some circumstances a train may be permitted to go past a restricted clearance at very slow speed.

The term can also be applied to the minimum size of road tunnels, the space beneath overpasses and the space within the superstructure of bridges, as well as doors into automobile repair shops, bus garages, filling stations, residential garages, multi-storey car parks, overhangs at drive-throughs and warehouses.

[citation needed]Eurocode 1: Actions on structures has a definition of "physical clearance" between roadway surface and the underside of bridge element.

[2] In UK, the "standard minimum clearance" for structures over public highways is 16 feet 6 inches (5.03 m).

[3] In United States, the "minimum vertical clearance" of overpasses on Interstate Highway System is 16 feet (4.9 m).

Increasing the structure gauge for a larger loading gauge can involve substantial work. The UK's Midland Main Line being upgraded in 2014.
In narrow curves, long cars like this 26.4 m (86 ft 7 in) long express train car protrude further into the gauge than on a straight track. (180° curve near Königstein im Taunus )