Increasing the structure gauge for a larger loading gauge can involve substantial work. The UK's Midland Main Line being upgraded in 2014.

A structure gauge, also called the minimum clearance outline, is a diagram or physical structure that sets limits to the extent that bridges, tunnels and other infrastructure can encroach on rail vehicles. It specifies the height and width of platforms, tunnels and bridges, and the width of the doors that allow access to a warehouse from a rail siding. Specifications may include the minimum distance from rail vehicles to railway platforms, buildings, electrical equipment boxes, signal equipment, third rails or supports for overhead lines.[1]

A related but separate gauge is the loading gauge: a diagram or physical structure that defines the maximum height and width dimensions in railway vehicles and their loads. The difference between these two gauges is called the clearance. 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.

Road traffic application

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.


A truck damaged by striking a railway bridge in Saltney in Cheshire in 2018

Motor vehicles hit railway bridges 1789 times in 2019 in the UK, where such incidents are known as bridge strikes, with several bridges being hit over 20 times in a single year. The total cost borne by the state was around £23 million.[2]

A 2.7 m (8 ft 10+14 in) high overpass bridge near St Petersburg, Russia, is known as the "Bridge of Stupidity" because it is often struck by vehicles despite many warning signs. In May 2018, after it was struck for the 150th time by a GAZelle truck, a birthday cake was presented to the bridge. This made national news.[3][4]

Similarly, an 11 ft 8 in (3.56 m) overpass in Durham, North Carolina, US, was frequently struck by vehicles, and made the news a number of times until it was raised in 2019.[5]

Infrared sensors, which trigger warning signs when a high vehicle approaches, were added to an underpass in Frauenfeld, Switzerland, only after several incidents.[6][7]

A similar situation exists at an underpass on Guy Street in Montreal, which has a clearance of 3.75 m (12 ft 4 in).[8][9][10]


See also


  1. ^ "Structure Gauge and Kinematic Envelope".
  2. ^ "Wise up size up: Don't try it for size – know your height before you go". Network Rail. Retrieved 31 August 2021.
  3. ^ Plavskaya, Yelena (May 27, 2018). "Петербуржцы подарили «мосту глупости» юбилейный торт" [Petersburgers Give “Bridge of Stupidity” a Birthday Cake]. Izvestiya.
  4. ^ "150th Truck Gets Stuck Under St. Petersburg's 'Bridge of Stupidity'". The Moscow Times. May 28, 2018.
  5. ^ "In the news". 24 April 2019.
  6. ^ "Unterführung wird jetzt deppensicher gemacht" [Underpass is Deepened]. 20 Minuten.
  7. ^ Hilzinger, Stefan. "Kanton und Stadt entschärfen die Frauenfelder Deppenfalle" [Canton and City Defuse the Frauenfelder Deppenfalle]. St. Galler Tagblatt.
  8. ^ Impact! New warning system on Guy St. underpass aims to reduce truck accidents, AMT train delays
  9. ^ IMPACT in action
  10. ^ Truck removed