The structure gauge, also called the minimum clearance outline, is the minimum height and width of tunnels and bridges as well as the minimum height and width of the doors that allow a rail siding access into a warehouse. In addition, the term may apply to the minimum distance to railway platforms (passenger or freight), buildings, electrical equipment boxes, railway signal equipment, third rails or to supports for overhead lines from the track. The width of a narrow cut can also affect the maximum loading gauge.
The difference between the structure gauge and the loading gauge is called the "clearance". The amount of clearance between the loading gauge and the structure gauge depends on the speed of the train, due to the train wobbling, so a train may be able to get past a restricted clearance by travelling at slow speed.
The term can also be applied to the minimum size of road tunnels and bridges, as well as doors into automobile repair shops, bus garages, filling stations, residential garages, multi-storey car parks and warehouses.
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