Noun
(rail transport, UK) The maximum permitted height and width for a rail vehicle, governed by clearances in tunnels, under bridges and past lineside structures.
(rail transport) A physical structure, often a metal frame above a railroad track, to check the maximum height and width of a vehicle.
Source: en.wiktionary.orgIn addition to the rail gauge, the tracks will be laid to conform with a Loading gauge which defines the maximum height and width for railway vehicles and their loads to ensure safe passage through bridges, tunnels and other structures. Source: Internet