Whyte Notation
A system for describing steam locomotive wheel arrangements as a sequence of three numbers (leading-driving-trailing wheels), e.g. 4-6-2 for a Pacific.
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Whyte notation is the system for describing steam locomotive wheel arrangements as a sequence of numbers separated by hyphens, representing in order: the leading (pony) wheels, the driving wheels, and the trailing wheels — each counted on both sides of the locomotive. A 4-6-2, for example, has four leading wheels (a 4-wheel pony truck), six driving wheels (three driving axles, one per side), and two trailing wheels (a 2-wheel truck behind the firebox). The arrangement was proposed by Frederick Whyte in a 1900 article for American Engineer and railroad Journal and rapidly displaced the earlier letter-based and prose-based descriptions.
Common arrangements acquired names attached to the type or the railroad that first popularised them. A 4-6-2 is a Pacific (first built for New Zealand Government Railways but the name comes from the type's role on the Atlantic-to-Pacific transcontinentals). A 4-8-4 is a Northern (Northern Pacific Railway, 1926). A 2-8-2 is a Mikado (sold in quantity to Japan). A 4-6-4 is a Hudson. A 4-8-8-4 is a Big Boy. Each name carries a body of operational and aesthetic association: Pacifics were fast passenger locos, Mikados were freight workhorses, Big Boys were Union Pacific's articulated heavy-haul climbers.
Articulated steam locomotives (those with two sets of driving wheels under a single boiler, hinged for tight curves) use a four-element notation: 2-8-8-2 means a 2-wheel pony, then two sets of eight drivers in the articulated frames, then a 2-wheel trailing truck. Triple articulateds — extremely rare — extend to five elements. Tank locomotives (which carry their water in side tanks rather than a separate tender) add a "T" suffix: 0-6-0T is a six-coupled saddle-tank.
The notation lost much of its everyday utility when diesel-electric and electric locomotives took over and adopted their own classification systems (UIC for Europe, AAR for North America). But Whyte numbers remain the standard reference for any steam locomotive, and even decades into the diesel era a Pacific is still a Pacific and a Big Boy is still a Big Boy.