A duplex locomotive is a steam locomotive that divides the driving force on its wheels by using two pairs of cylinders. With one pair of cylinders, set at 90° apart to prevent a dead spot, there is an inbalance caused by the reciprocating action of the pistons and valve gear. Though some of this can be countered by weights on the wheels, this unbalances the wheels and results in the track being pounded, a phenomenon called hammer blow. An additional pair of cylinders and pistons therefore provide drive at each 90° arc and improve the balance and ride. Configurations with pistons side by side or one above the other have been tried, as have two independent sets of driving wheels, each being driven by a pair of cylinders. Great Western Railway No. ...

A typical American single-engine steam locomotive, such as a 4-8-4, would have two large cylinders powering the driving wheels. All driving axles are coupled together. A 4-4-4-4 locomotive is the 4-8-4's duplex equivalent. One pair of cylinders powers each pair of driving axles. While a 4-4-4-4 could certainly be built as an articulated locomotive, duplex locomotives as a rule had rigid frames. Using a duplex drive meant that the pistons could be smaller and the side rods lighter. This reduced stress on both locomotive parts and the track itself. However, many duplex locomotives had adhesion problems and were prone to wheelslip. The extra pair of cylinders made maintenance more difficult as well. The Norfolk & Western Railways Class J locomotive #611, a 1950 product of the railroads own Roanoke, Virginia shops. ... This article needs to be wikified. ... On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotives pistons (or turbine, in the case of a steam turbine locomotive). ... A 4-4-4-4 steam locomotive, in the Whyte notation for describing locomotive wheel arrangements, has four leading wheels in a leading truck, two sets of four driving wheels, and a four-wheel trailing truck. ... An articulated locomotive is a steam locomotive with one or more engine units which can move relative to the main frame. ... A locomotive frame is the structure that forms the backbone of the railway locomotive, giving it strength and supporting the superstructure elements such as a cab, boiler or bodywork. ... Railroad or railway tracks are used on railways, which, together with railroad switches (points), guide trains without the need for steering. ...

The Pennsylvania Railroad used duplex locomotives extensively, but they often had short service lives. The Pennsylvania Railroad (AAR reporting mark: PRR) was an American railroad existing 1846–1968, after which it merged into Penn Central Transportation. ...