In Whyte notation

Whyte notation

The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early twentieth century encouraged by an editorial in American Engineer and Railroad Journal...

, a 2-2-4 is a railroad steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

 that has two leading wheel

Leading wheel

The leading wheel or leading axle of a steam locomotive is an unpowered wheel or axle located in front of the driving wheels. The axle or axles of the leading wheels are normally located in a truck...

s followed by two coupled driving wheel

Driving wheel

On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...

s and four trailing wheel

Trailing wheel

On a steam locomotive, a trailing wheel or trailing axle is generally an unpowered wheel or axle located behind the driving wheels. The axle of the trailing wheels was usually located on a trailing truck...

s. This was an unusual wheel arrangement, only used on a few specialised locomotives.

Equivalent classifications

Other equivalent classifications are:

• UIC classification
  UIC classification
  The UIC classification of locomotive axle arrangements describes the wheel arrangement of locomotives, multiple units and trams. It is set out in the International Union of Railways "Leaflet 650 - Standard designation of axle arrangement on locomotives and multiple-unit sets". It is used in much...
  
  : 1A2 (also known as German classification and Italian classification)
• French classification: 112
• Turkish classification
  Turkish classification
  In the Turkish classification system for railway locomotives, the number of powered axles are followed by the total number of axles. It is identical to the Swiss system except that the latter places a slash between the two numbers.Thus0-6-0 becomes 33...
  
  : 14
• Swiss classification: 1/4

UK examples

In the United Kingdom, the North Eastern Railway

North Eastern Railway (UK)

The North Eastern Railway , was an English railway company. It was incorporated in 1854, when four existing companies were combined, and was absorbed into the London and North Eastern Railway at the Grouping in 1923...

, had four tank locomotive

Tank locomotive

A tank locomotive or tank engine is a steam locomotive that carries its water in one or more on-board water tanks, instead of pulling it behind it in a tender. It will most likely also have some kind of bunker to hold the fuel. There are several different types of tank locomotive dependent upon...

s of this wheel arrangement, all of which had previously been rebuilt from other types. They were: No. 66 Aerolite

NER 66 Aerolite

North Eastern Railway No. 66 Aerolite is a preserved British steam locomotive. It was classified X1 by the LNER.Aerolite was built in 1869 as a replacement for an engine of the same name built by Kitson's for the Great Exhibition in 1851 and which was destroyed in a collision in 1868...

 rebuilt as a 2-2-4T in 1902 and later known as in LNER class X1; No. 957, which had been rebuilt from a BTP class 0-4-4T

NER Bogie Tank Passenger

The North Eastern Railway Bogie Tank Passenger locomotives were designed by Edward Fletcher in 1873. The locomotives were for hauling passenger services on branch lines. They had an 0-4-4 wheel layout and were later designated as G6. A total of 124 G6s were built.With the introduction of the NER...

 in 1903 and later classified as X2 class. NER 190 Class, later class X3 had two members, nos. 190 and 1679, both rebuilt from 2-2-2

2-2-2

Under the Whyte notation for the classification of steam locomotives, 2-2-2 represents the wheel arrangement of two leading wheels on one axle two powered driving wheels on one axle, and two trailing wheels on one axle. The wheel arrangement both provided more stability and enabled a larger firebox...

 tender locomotive

Tender locomotive

A tender or coal-car is a special rail vehicle hauled by a steam locomotive containing the locomotive's fuel and water. Steam locomotives consume large quantities of water compared to the quantity of fuel, so tenders are necessary to keep the locomotive running over long distances. A locomotive...

s. All four were inherited by the London and North Eastern Railway

London and North Eastern Railway

The London and North Eastern Railway was the second-largest of the "Big Four" railway companies created by the Railways Act 1921 in Britain...

 (LNER) at the time of its formation on 1 January 1923, and withdrawn from service between 1931 and 1937. No. 66 Aerolite

NER 66 Aerolite

North Eastern Railway No. 66 Aerolite is a preserved British steam locomotive. It was classified X1 by the LNER.Aerolite was built in 1869 as a replacement for an engine of the same name built by Kitson's for the Great Exhibition in 1851 and which was destroyed in a collision in 1868...

 has been preserved at the National Railway Museum

National Railway Museum

The National Railway Museum is a museum in York forming part of the British National Museum of Science and Industry and telling the story of rail transport in Britain and its impact on society. It has won many awards, including the European Museum of the Year Award in 2001...

 in York

York

York is a walled city, situated at the confluence of the Rivers Ouse and Foss in North Yorkshire, England. The city has a rich heritage and has provided the backdrop to major political events throughout much of its two millennia of existence...

.