Gauge

Gauge is the inside-to-inside distance between the two running rails of a railway track, measured at a defined distance below the railhead (in most systems, 14 mm). The dominant world standard — known as standard gauge — is 1435 mm, a dimension adopted by George Stephenson from the colliery wagonways of north-east England in the 1820s and subsequently inherited by most of the railway networks that grew from British engineering exports. Roughly 60% of the world's railways are standard gauge today.

The other notable gauges fall into two camps. Broad gauges run wider than standard — 1520 mm in Russia and most of the former Soviet bloc, 1668 mm in Iberia, 1600 mm in Ireland, 1676 mm in much of India and Argentina — and were typically chosen for greater vehicle stability or for political reasons to prevent invasion by neighbours. Narrow gauges run narrower — 1067 mm (Cape gauge), 1000 mm (metre gauge), 762 mm (the Indian narrow), 600 mm (the Decauville military and industrial gauge) — and were chosen for cheaper construction over difficult terrain, tighter curves, and lower axle loads.

Where two gauges meet, three engineering solutions exist. Break-of-gauge stations require passengers to change trains and freight to be transhipped, historically a major operational drag (still the standard at most Russian-Chinese border crossings). Dual gauge lays a third rail inside or outside the original two, allowing both gauges to use the same alignment at the cost of a complicated switching geometry. Gauge-changing rolling stock — Talgo's variable-gauge sets in Spain, the Japanese Free Gauge Train prototypes — physically slide their wheels along the axle through a wayside changer in under a minute.

For railfans, a narrow-gauge train is instantly recognisable even at a distance: it sits closer to the ground, the locomotives are stubbier, and the carriages have a doll-house quality that broad gauge can never reproduce.