4-6-2

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.
The equivalent UIC classification is '2'C1''.

Contents

History

Origin of the name

Regional variations

Australia

Austria

Finland

Germany

New Zealand

North America

South Africa

United Kingdom

Cultural references

References

History

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 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 wanting 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 built the first steam locomotive to reputed to deliver 1 hp of power for every 100 lb of its weight, the New Zealand-designed and built Pacific, A^B class.^[1]
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 were 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" that used an extra pair of driving wheels to deliver more traction, and thus transfer more power to the rail.

Origin of the name

The origin of the name "Pacific" for this type of locomotive 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 later than 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 adopted reflected different approaches. The SAR 600 class reflected contemporary American locomotive practice both in design features and appearance, with two large 24 x 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.^[2]

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 no. 1089, built in 1956, is the last main-line steam locomotive built in Australia.^[3]

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

Twenty-two Pacifics classified 'Hr1' and known by the nickname '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 from them.
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 between 1.8 and 2.2 m high). 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 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 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

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 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 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 (they claimed 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 today is operated 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 Class 4-6-2 steam locomotive as part of its collection. The engine 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".

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 (1828 mm) in diameter and had Poppet valve gear. These six locomotives were in service from 1935, and one engine achieved 85 mph These locomotives retired 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. A second smaller Class 10 locomotive of the 4-6-2 arrangement operated on the SAR from 1910.

United Kingdom

4-6-2s were built by all of the four pre-BR railway companies (although the Great Western Railway built only the one example, also 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 officially recorded British locomotive to reach 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 also built some fast Pacific locomotives, with the Princess Coronation Class briefly holding 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.
After 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.
After the formation of British Railways in 1948, plans were drawn up for new standard locomotive classes, including the Standard Class 7 Britannia Pacifics. These locomotives 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 used many parts common with the Britannias, but had three cylinders and Caprotti valve gear.^[4]

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

★ Soo Line 2-8-2 back in steam, Gilchinski, Steve, , , Trains magazine, 1997

★ 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

1. TrainWeb - Ab class retrieved 9 October 2006
2. S class locomotive diagram retrieved 30th September 2006
3. QR Corporate - Heritage Services retrieved 30 September 2006
4. Duke of Gloucester Website - A Concise History of the Locomotive retrieved 9 October 2006