4-6-2

Blue Mountain and Reading Railroad 425 gets ready for the day's tourist train in 1993.

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). These locomotives are also known as Pacifics. On many railways, Pacific steam locomotives provided the motive power for express passenger trains throughout much of the early to mid 20th century before being replaced by diesel and electric locomotives and high speed trains.

Other equivalent classifications are:
UIC classification: 2C1 (also known as German classification and Italian classification)
French classification: 231
Turkish classification: 36
Swiss classification: 3/6

The equivalent UIC classifications are refined by 2'C'1' or 2'C1' depending on the mounting of the final axle.

History

The NZR Q class, the first Pacific locomotive.

The success of the 4-6-2 design can be attributed its four-wheel leading truck, which made for stability at speed; six driving wheels, which allowed for the application of more power than 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.

New Zealand Railways (NZR) were the first major railway to purchase and operate 4-6-2 locomotives, in 1901 placing an order for 13 Q class locomotives with the Baldwin Locomotive Works of Philadelphia, Pennsylvania. The NZR Chief Mechanical Engineer, A. L. Beattie, ordered the Q class as a locomotive with a large firebox that would be able to burn poor lignite coal from eastern South Island mines efficiently. While there were some examples of Pacifics built prior to 1901, these were rebuilds of 4-6-0s rather than true Pacifics built to take full advantage of the design. By 1915 the NZR had designded and built the first steam locomotive reputed to deliver 1 hp of power for every 100 lb of its weight, the A^B class.

While the Pacific remained the major express passenger locomotive type on many railways, being built in the UK, for example, well into the 1950s, elaborations of the design were deemed necessary for certain situations. The most notable of these was the 4-6-4 "Hudson", which had a four-wheel trailing truck that permitted a larger firebox (though at the loss of adhesive weight); and the 4-8-2 "Mountain", which used an extra pair of driving wheels to deliver more power to the rails.

Origin of the name

The origin of the name "Pacific" is that the first locomotives of the type were designed and built for railways in New Zealand and consequently shipped from their manufacturer in the United States across the Pacific Ocean to New Zealand.

An alternative explanation of the name derives the term "Pacific" from early examples of the type that were built after the NZR Q class for the Missouri Pacific Railroad.

Regional variations

Australia

In the late 1920s, heavy Pacific locomotives were introduced by both South Australian Railways and Victorian Railways in response to increasingly heavy passenger trains and demand for faster services. Although similar in size, power, and top speed, the designs reflected different approaches. The SAR 600 class reflected contemporary American locomotive practice both in design features and appearance, with two large 24 × 28 in (610 x 710 mm) cylinders. The VR S class showed a strong British LNER influence, with three 20½ x 28 in (521 x 710 mm) cylinders, and Gresley conjugated valve gear driving the third (inside) cylinder.

NSWGR C38 class 3801

The New South Wales Government Railways introduced the C38 class for express passenger service in 1943. These two-cylinder Pacifics had a free-steaming 245 psi boiler and were renowned for their performance. Class leader 3801 has achieved considerable fame in preservation, with notable feats such as a transcontinental journey from Sydney to Perth in 1970.

In the post-war era, Pacifics were also introduced on narrow-gauge Australian railways with the Tasmanian Railways M class and Queensland Railways BB18¼ class. BB18¼ 1089, built in 1956, was the last main-line steam locomotive built in Australia.

Austria

The only example in Austria was the class 629 tank locomotive, built from 1913. This highly successful type remained in service until 1975.

Finland

A Finnish "Ukko-Pekka" class Hr1.

Twenty-two Pacifics classified Hr1 and nicknamed Ukko-Pekka, after Finnish President Pehr Evind Svinhufvud, 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 took over.

Two Hr1s remain in operational condition, Hr1 1021 owned by the VR Group and Hr1 1009 privately owned.

Germany

All German 4-6-2s were express passenger locomotives with large driving wheels (average diameter between 1.8 and 2.2 m). They were the BR 01 (two-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 BR 18 (pre WWI locomotives of the different state railway companies), with many subclasses (e. g. BR 18.4, formerly S 3/6 from Bavaria). Some were streamlined, such as the BR 01.10 and BR 03.10 subclasses, with three cylinders instead of two plus streamlining, increasing maximum speed).

New Zealand

See History section above.

North America

The Pennsylvania Railroad's class K4s, a well known 4-6-2 type.

File:DSC02328.JPG

Baldwin Pacific Class built in 1920 for Reid Newfoundland Company

Pacifics were the predominant steam passenger power in North America during the first half of the 20th century. Few railroads did not roster 4-6-2 locomotives as premier passenger power, although they were later supplanted in top-flight service on many roads on by larger 4-6-4 "Hudson", 4-8-2 "Mountain" or 4-8-4 "Northern" locomotives as train weights increased.

Approximately 7000 Pacifics were produced for U.S. and Canadian railroads. The largest user was the Pennsylvania Railroad, which had 697, including 425 class K4s, the largest single class of locomotive built in the United States (claimed to be worldwide, but this is unlikely, given huge fleets of identical German, Russian, and Chinese locomotives).

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 operates for excursion runs primarily in the Midwest United States.

The Humbermouth Historic Train Site in Corner Brook, Newfoundland, Canada includes a non-functional Baldwin Pacific as part of its collection. It was built in 1920 in Philadelphia, acquired by the Reid Newfoundland Company Ltd., and later transferred to the Newfoundland Railway. It was originally designated "193", later "593".

Russia/Soviet Union

Pacifics were not common in Russia. The only known example was the 4-cylinder Lp-class express passenger locomotive built by Putilov works for Vladikavkaz private railroad. Their chief designer was Vazlav Lopushinskii, who later emmigrated from Soviet Russia. These locomotives were the most powerful passenger locomotives in tsarist Russia.

South Africa

South Africa with its 3 ft 6 in (1065 mm) gauge network has the distinction of operating a 4-6-2 locomotive with the biggest driving wheels on the narrow gauge. The Class 16E variant drivers were 6 ft (1.828 m) in diameter and had Poppet valve gear. The six locomotives were in service from 1935, and one achieved 85 mph (137 km/h) These locomotives were withdrawn in 1975, though preserved examples operated on specials until the end of steam in 1990. The class had the highest boilers on South African rails, with the centre-line 9 ft 3 in (2.8194 m) above rail level.

The other Class 16 variants with 4-6-2 drivers were not as large or as fast as the Es. The smaller Class 10 locomotive of the 4-6-2 arrangement operated on the SAR from 1910.

United Kingdom

The Mallard, holder of the world speed record for steam traction.

4-6-2s were built by all of the four pre-BR railway companies (although the Great Western Railway built only one, the first Pacific in Great Britain, No. 111 The Great Bear). A number of British Pacifics were notable for setting official world speed records for steam traction.

The LNER built some of the fastest and most famous examples. The GNR A1 Class (later rebuilt into the A3) featured three cylinders and the innovative Gresley conjugated valve gear, and No. 4472 Flying Scotsman was the first British locomotive to be officially recorded as reaching 100 mph (161 km/h). This speed was surpassed by the streamlined A4 class, with record performances by No. 2509 Silver Link (reaching 112mph (180km/h) on its inaugural run in 1935) and No. 4468 Mallard touching 126 mph (203 km/h) on 3 July 1938, which is still the world speed record for steam traction.

The London, Midland and Scottish Railway's Princess Coronation Class briefly held the British speed record for steam traction when No. 6220 Coronation reached 114 mph (183 km/h) on 29 June 1937. The LMS Princess Royal Class was also used as the basis for an unusual experimental locomotive, the Turbomotive.

During World War II the Southern Railway introduced a number of highly innovative designs, including the Merchant Navy Pacific and the West Country and Battle of Britain Classes.

British Railways Standard Class 7 Britannia Pacifics were a simple expansion two-cylinder design with Walschaerts valve gear, their conservative design reflecting a requirement for a more cost-effective, lower maintenance locomotive. The final Pacific design in the UK was No. 71000 Duke of Gloucester, built in 1954. It had many parts in common with the Britannias, but with three cylinders and Caprotti valve gear.

Cultural references

Swiss composer Arthur Honegger wrote his symphonic poem Pacific 231 in honor of the locomotive, using the French designation 2-3-1, counting by axle instead of wheel.

References

• Gilchinski, Steve (1997). "Soo Line 2-8-2 back in steam". Trains magazine. 57 (2): 24–25. {{cite journal}}: Cite has empty unknown parameter: |1= (help); Unknown parameter |month= ignored (help)
• Heath, Eric, and Stott, Bob; Classic Steam Locomotives of New Zealand, Grantham House, 1993.

South African references

• RSA Government, South African Last Stronghold of Steam, SAR, Kimberley, 1978. ISBN 0 949934 24 0
• Lewis, C.P. & Jorgensen. The Great Steam Trek, Struiker, Cape Town, 1978. ISBN 0 86977 101 9

Specific