The wheel arrangement 4-2-0 designates a steam locomotive in which there is a leading, two-axle bogie and a driving axle. It must be distinguished from the Crampton type with fixed carrying axles, i.e. the UIC wheel arrangement 2A or 3A instead of 2'A. In different countries, this wheel arrangement is referred to as follows:
Jervis
UIC: 2'A
Whyte: 4-2-0
Switzerland: 1/3
France: 210
Turkey: 13
In the early days of the railway, a large proportion of the locomotives had a 2-2-0 wheel arrangement, which satisfied the traction requirements of the time. While the tracks in England were laid carefully and the terrain was often cut or filled up to compensate for differences in height, in North America the tracks were simply laid onto the existing ground level for reasons of cost. This led to an uneven and winding track, for which the existing locomotives had a too rigid chassis.
The solution was to replace the carrying axle with a movable bogie that could adapt to the inclines and curves. The first locomotive with this arrangement was the “Brother Jonathan”, which John B. Jervis had developed for the Mohawk & Hudson. This also led to the name “Jervis” used in North America for this wheel arrangement. One of the two variants of this wheel arrangement had the driving axle behind the firebox and was mainly built by Baldwin. It was characterized by a very smooth running, but had a low adhesive weight, since more weight was on the bogie. Norris' preferred variant had the firebox located behind the driving axle, which increased the load on the driving axle but resulted in less smooth running.
It was in the USA in particular, where this design was created, that it gained the greatest popularity and was built in large numbers from the 1830s to the 1850s. Many locomotives with the wheel arrangement 0-4-0 were even subsequently rebuilt to 4-2-0, since the running characteristics were more important than the adhesive weight at the time. They were later replaced by locomotives with a 4-4-0 wheel arrangement, since a second coupled axle had become necessary. Some locomotives with a 4-2-0 wheel arrangement were also used in Europe, but disappeared again in the 1840s. In Great Britain they were soon replaced by similar locomotives with three fixed axles, from which the Crampton type developed.