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Under the Whyte notation for the classification of steam locomotives, a 4-6-2 locomotive has four leading wheels (generally arranged in a leading truck), six coupled driving wheels and two trailing wheels (often but not always in a trailing truck).
The equivalent UIC classification is 2'C1'.
History
In the United States and Britain, the 4-6-2 locomotive type was named the Pacific as the first locomotives of the style were shipped across the Pacific Ocean to New Zealand. A famous 4-6-2 locomotive is the LNER A4 class 4468 Mallard.
New Zealand Railways (NZR) was the first to purchase and operate a proper 4-6-2 wheel arrangement in 1901, with the order for 13 Q class locomotives being placed with Baldwin. Upon completion, the locomotives had to be shipped across the Pacific Ocean to New Zealand, hence the name. The reason for the design was that a locomotive with a large firebox was required to burn poor lignite coal from the South Island. So it was the NZR Chief Mechanical Engineer, A. W. Beattie, who ordered the first true Pacific. There are some known examples of Pacifics prior to 1901, but these were rebuilds of 4-6-0s and were not designed as Pacifics at the time of their original construction. In 1915 NZR built the (reputedly) first steam locomotive to develop 100 hp for 100 lb of engine weight. It was a New Zealand built and designed Pacific and was designated the Ab class.
It should be noted that some sources incorrectly state that the term Pacific stems from early examples of the 4-6-2 type of locomotive being built for the Missouri Pacific Railroad. The correct explanation of the name is the New Zealand one.
Pacifics were the predominant steam passenger power in America in the twentieth century. Few railroads did not roster 4-6-2 locomotives as premier passenger power, although they were supplanted in top-flight service on many roads later on by larger 4-6-4 "Hudson", 4-8-2 "Mountain" or 4-8-4 "Northern" locomotives as train weights increased.
Approximately 7000 locomotives of this wheel arrangement were produced for U.S. and Canadian railroads. The largest user was the Pennsylvania Railroad, which had a total of 697 Pacifics, including 425 class K4s, the largest single class of locomotive ever built in the United States (they claimed worldwide, but this is unlikely to be true given huge fleets of identical German, Russian, and Chinese locomotives).
The success of the 4-6-2 design can be attributed to the presence of a four-wheel leading truck, which made for stability at speed; six driving wheels which allowed for the application of more power compared to the earlier 4-4-2 "Atlantic" design; and a two-wheel trailing truck which permitted the firebox to be behind the high driving wheels, allowing it to be both wide and deep. On a locomotive without a trailing truck such as a 4-6-0 "ten wheeler", the designer is caught in a dilemma; the firebox can either fit between the driving wheels and be narrow and deep, or it can fit above the driving wheels and be wide and shallow.
The Pacific was further developed into the 4-6-4 "Hudson", with a four-wheel trailing truck permitting a still larger firebox, and into the 4-8-2 "Mountain", with an extra pair of driving wheels giving more traction, and thus able to transfer more power.
One notable 4-6-2 was preserved and has been restored to operating condition for excursions. Soo Line 2719 hauled the last of Soo Line Railroad's steam-powered trains in 1959. It was restored beginning in 1996 and today is operated for excursion runs primarily in the Midwest United States.
Germany
All German 4-6-2s were express passenger locomotives with large driving wheels (average between 1.8 and 2.2 m high). They were the BR 01 (twin cylinder standard type of Deutsche Reichsbahn, BR 02 (four cylinder compound prototype), BR 03 (lighter version of BR 01), BR 10 (only two prototypes built by Deutsche Bundesbahn and the BR 18 (pre WWI locomotives of the different state railway companies) with its many subclasses (e. g. BR 18.4, formerly S 3/6 from Bavaria). Some were streamlined, such as the subclasses BR 01.10 and BR 03.10, with three cylinders instead of two plus streamlining, increased maximum speed).
Austria
The only application of this wheel arrangement in Austria was in tank locomotives of class 629, built from 1913 on. This highly successful type remained in service until 1975.
Finland
Twenty-two Pacifics (classified Hr1 and known by the nickname "Ukko-Pekka") were constructed in Finland by Tampella and Lokomo between 1939 and 1957. They were the largest passenger locomotives built or used in Finland and remained the primary locomotives used for express trains until 1963, when diesels overtook them.
Two Hr1s remain in operational condition, one (Hr1 1021) owned by the VR Group, and the other (Hr1 1009) privately owned.
References
- Heath, Eric, and Stott, Bob; Classic Steam Locomotives Of New Zealand, Grantham House, 1993
"4-6-2"