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...

, 2-4-4-2 refers to 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 four 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, a second set of four coupled driving wheels, and two 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.

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...
  
  : 1BB1 (also known as German classification and Italian classification)
• French classification: 120+021
• 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...
  
  : 23+23
• Swiss classification: 2/3+2/3

The 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...

 is refined to (1'B)B1' for a Mallet locomotive

Mallet locomotive

The Mallet Locomotive is a type of articulated locomotive, invented by a Swiss engineer named Anatole Mallet ....

.

US Examples

This articulated wheel arrangement was rare in North America. Most were built as logging locomotives, presumably to better negotiate the uneven (and often temporary) trackwork that characterized such operations. The added mechanical complexity was found to be of dubious value, as reflected in their limited production and use.

Number 7

Built by ALCO, this locomotive is a Mallet Compound engine, built for the Taupo Totara Timber Co for use on their 51 mile private tramway system, the Mokai Tramway between Putaruru

Putaruru

Putaruru is a small town in the Waikato region of New Zealand's North Island. It is on the Oraka River 65 kilometres south-east of Hamilton....

 and Mokai in the North Island. It is now preserved on the Glenbrook Vintage Railway

Glenbrook Vintage Railway

The Glenbrook Vintage Railway is a steam railway in Glenbrook, New Zealand. One of New Zealand's premiere rail heritage sites, it is not a museum as such, but rather a fully self-supporting, operating steam railway, built almost entirely by volunteer labour...

, near Auckland, New Zealand and now carries the number 4. The engine is currently out of service awaiting overhaul, but can still be seen at the railway's Pukeoware workshops.