loading gauge

noun2

1. •
   (UK) The maximum permitted height and width for a rail vehicle, governed by clearances in tunnels, under bridges and past lineside structures.

   • The first "K" class 4-8-4 appeared in 1932. Designed and built in New Zealand, these are probably the biggest non-articulated locomotives possible within the loading gauge. — 1954
2. •
   A physical structure, often a metal frame above a railroad track, to check the maximum height and width of a vehicle.