The ability to move a railway vehicle and its load along the railway tracks depends on the height and width profile, known as the loading gauge, of the route concerned. A railway vehicle must comply with the route loading gauge to ensure that it passes clear of all structures, principally over-bridges and tunnels but also features such as station platforms, canopies and overhead or lineside equipment.

The high tech laser and light technology of the Structure Gauging Train (SGT) provide a superb picture of such infrastructure. The laser beams reflect back to the train, and should the distance that the laser travels be too small, an alert isactivated. The train also measures the distance between pairs of tracks ensuring that passing trains will not foul the loading gauge
The recent SGT had its beginnings in the mid-1970's. Several experiments were carried out utilising the four-wheeled Wickham self-propelled laboratory (sometimes known as the Elliott Track recording car owing to its equipment being supplied by Elliott Automation Ltd) and also as the 'Yellow Peril'. Using a dual-triangulation system  with lights and cameras this optical method was a big improvement over the existing ways of gauging. It was trialled at the Old Dalby test track.

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The work pioneered at Old Dalby with the Wickham Car led the way for the building of the special Structure Gauging Train in the early 1980's. It was completed for the Director of Civil Engineering in the summer of 1986, the project having been in the development stage for many months.

A specially-built four wheel structure-gauging optical vehicle, ZXQ DC460000 based on a steel carrier chassis, was formed as the middle car of a 3-vehicle formation. The two outer coaches were RDB975081 (formerly Test Coach Hermes converted to a driving trailer vehicle and re-numbered DB975081) and RDB975280 - (formerly Test Coach Mercury) became DB975280 and housed computer data collection and staff facilities. The two vehicles had previously been used extensively by the Train Control Group at the Mickleover test track and elsewhere from the early 1970's. 

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The SGT on trial at the Old Dalby test track in 1985 with DB975081 nearest the camera, then DC460000 and DB975280 at the back. Note that the leading vehicle is fitted with Commonwealth bogies

Following major changes within the Research Division in the late 1980's the SGT was transferred to the DM& EE and then to Railtest and finally to Network Rail.

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Another close up of DC460000 - the optical car - now using laser technology

Last seen at York Klondyke Sidings on 22 July 2017

Last seen at York Klondyke Sidings on 22 July 2017

This vehicle was scrapped at Sims Metals, Beeston in February 2015