4-6-2

A 4-6-2 is a steam locomotive whose wheels are arranged, from front to back, as a four-wheel leading truck, six coupled driving wheels, and a two-wheel trailing truck. The Whyte notation describes this by counting axles, not wheels: 4 leading, 6 driving, 2 trailing. This arrangement became the dominant express passenger design on North American and European railways from the 1910s through the 1950s.

The four leading wheels (two axles) swivel as a unit, helping the locomotive navigate curves while the engineer maintains speed. The six driving wheels, all connected by rods and fed steam from the boiler, provide the tractive force. The trailing truck, with just two wheels, carries the weight of the firebox and allows a larger, longer boiler than a 4-4-2 (Ten-Wheeler) could support. This geometry lets builders pack more heating surface and steam capacity into the same length.

The type became known as the Pacific, a name first applied to locomotives built for the Pacific branch of the Missouri Pacific Railroad in 1902. The 4-6-2 dominated North American passenger service: the New York Central, Pennsylvania Railroad, Chesapeake and Ohio, Southern Pacific, and many others operated hundreds. European railways adapted the design, often with higher boiler pressures (up to 250 psi or more) and smaller cylinders to suit tighter clearances and sharper curves.

Power and speed trade-offs

A typical American 4-6-2 might produce 40,000 to 50,000 pounds of tractive effort at starting, with a boiler pressure around 200 to 210 psi. Running at track speed (often 80 to 100 mph on level grades), a Pacific could haul 12 to 16 cars. The design was responsive, steaming easily, and much lighter per unit of power than the six-coupled 2-6-2 Mikado type intended for freight.

The trailing truck was also a mechanical vulnerability: it carried weight without helping to drive the locomotive, and the pivot bearings could wear, introducing play that reduced steering precision on curves. Many railroads fitted boosters (small engines) to the trailing axle to increase tractive effort when starting, turning a 4-6-2 into a 4-6-4 in terms of actual driving force if not wheel count.

By the 1940s, streamlining was applied to many Pacifics, but even without it the type remained fast enough for the streamliners of the era. The last new Pacifics were built in the late 1940s; the last one in regular service ran in North America in the early 1960s. A few preserved examples still operate on heritage railways, and the 4-6-2 remains the most recognized silhouette of American steam locomotion.