Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement of four leading wheels on two axles, two powered driving wheels on one axle and no trailing wheels. This type of locomotive is often called a Jervis type, the name of the original designer.
Overview
The wheel arrangement type was common on United States railroads from the 1830s through the 1850s. The first to be built was the Experiment, later named Brother Jonathan, for the Mohawk and Hudson Railroad in 1832. It was built by the West Point Foundry based on a design by John B. Jervis. Having little else to reference, the manufacturers patterned the boiler and valve gear after locomotives built by Robert Stephenson of England. A few examples of Stephenson locomotives were already in operation in America, so engineers did not have to travel too far to get their initial ideas.
In England, the was developed around 1840 from the 2-2-2 design of Stephenson's first Long Boiler locomotive, which he had altered to place two pairs of wheels at the front to improve stability, with the outside cylinders between them.
In the United States, the design was a modification of the design, then in common use. The proved to be too rigid for the railroads of the day, often derailing on the tight curves and rapid elevation changes of early American railroads. For the , Jervis introduced a four-wheel leading truck under the locomotive's smokebox. It swiveled independently from the main frame of the locomotive, in contrast to the English engines which had rigid frames. The pistons powered a single driving axle at the rear of the locomotive, just behind the firebox. This design resulted in a much more stable locomotive which was able to guide itself into curves more easily than the .
This design proved so effective on American railroads that many of the early were rebuilt as . The excelled in its ability to stay on the track, especially those with the single driving axles behind the firebox, whose main virtue was stability. However, with only one driving axle behind the firebox, the locomotive's weight was spread over a small proportion of powered wheels, which substantially reduced its adhesive weight. On locomotives which had the driving axle in front of the firebox, adhesive weight was increased. While this plan placed more of the locomotive's weight on the driving axle, it reduced the weight on the leading truck which made it more prone to derailments.
Usage
South Africa
thumb|left|[[South African Dutton road-rail tractors|Dutton road-rail tractor, a modified Yorkshire steam tractor, c. 1924]]
In 1923, the South African Railways conducted trials with a prototype petrol-paraffin powered road-rail tractor and, in 1924, placed at least two Dutton steam road-rail tractors in service on the new narrow gauge line between Naboomspruit and Singlewood in Transvaal. The petrol-paraffin prototype and one of the latter had a wheel arrangement.
The prototype was a modified Dennis tractor which was fitted with a removable bogie between the front wheels to lift them high enough to prevent ground contact. A ball pin on the bogie fit into a socket in the front axle, and the bogie could easily be removed or replaced by running the tractor up a pair of ramps, placed on both sides of the track.
The production model was a modified Yorkshire steam tractor, fitted with jacks at the front to allow a separate bogie to be manoeuvred into position underneath the front axle to guide it on the rails. Without the bogie, the vehicle could still be driven on ordinary roads and had the advantage of being able to be detached and run around the train, without requiring special loops for that purpose. For reversing on the track, as when shunting, the rear wheels were modified to be steerable.
United States
thumb|The [[Chicago and North Western Transportation Company|Chicago and North Western Railway's first locomotive, Pioneer]]
The first railroad locomotive to operate in Chicago, Illinois was a , the Pioneer, which was built in 1837 by Baldwin Locomotive Works for the Utica and Schenectady Railroad in New York. It was later purchased used by William B. Ogden for the Galena and Chicago Union Railroad, the oldest predecessor of the Chicago and North Western Railway. The locomotive arrived in Chicago by schooner on 10 October 1848 and it pulled the first westbound train out of the city fifteen days later, on 25 October 1848.