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Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement of four leading wheels on two axles, six powered and coupled driving wheels on three axles and two trailing wheels on one axle. The locomotive became almost globally known as a Pacific type after a locomotive built by the Baldwin Locomotive Works in Philadelphia was shipped across the Pacific Ocean to New Zealand.

Overview

The introduction to New Zealand of the design in 1901 has been described as "a veritable milestone in locomotive progress". On many railways worldwide, Pacific steam locomotives provided the motive power for express passenger trains throughout much of the early to mid-20th century, before either being superseded by larger types in the late 1940s and 1950s, or replaced by electric or diesel-electric locomotives during the 1950s and 1960s. Nevertheless, new Pacific designs continued to be built until the mid-1950s.
The type is generally considered to be an enlargement of the Atlantic type, although its prototype had a direct relationship to the Ten-wheeler and Prairie, effectively being a combination of the two types. The success of the type can be attributed to a combination of its four-wheel leading truck which provided better stability at speed than a Prairie, the six driving wheels which allowed for a larger boiler and the application of more tractive effort than the earlier Atlantic, and the two-wheel trailing truck, first used on the New Zealand Prairie of 1885. This permitted the firebox to be located behind the high driving wheels and thereby allowed it to be both wide and deep, unlike the Ten-wheeler which had either a narrow and deep firebox between the driving wheels or a wide and shallow one above.
The type is well-suited to high speed running. The world speed record for steam traction of has been held by a British Pacific locomotive, the Mallard, since 3 July 1938.

Development

The two earliest locomotives, both created in the United States, were experimental designs which were not perpetuated. In 1887, the Lehigh Valley Railroad experimented with a Ten-wheeler design with a Strong's patent firebox, a cylindrical device behind the cab which required an extension of the frame and the addition of two trailing wheels to support it. In 1889, the Chicago, Milwaukee & St Paul Railway rebuilt a conventional with trailing wheels as a means of reducing its axle load.
In 1896, six Q class tank locomotives were introduced on the Western Australian Government Railways.
The first true Pacific, designed as such with a large firebox aft of the coupled wheels, was ordered in 1901 by the New Zealand Railways Department from the Baldwin Locomotive Works of Philadelphia, Pennsylvania. The NZR Chief Mechanical Engineer, Alfred Beattie, ordered thirteen new Q class locomotives with a sufficiently large Wootten firebox to efficiently burn poor grade lignite coal from eastern South Island mines. Even before they had completed the order from New Zealand, the Baldwin engineers realised the advantages of this new type, and incorporated it into standard designs for other customers. The design was soon widely adopted by designers throughout the world.

Origin of the name

There are different opinions concerning the origin of the name Pacific. The design was a natural enlargement of the existing Baldwin Atlantic type, but the type name may also be in recognition of the fact that a New Zealand designer had first proposed it. Usually, however, new wheel arrangements were named for, or named by, the railroad which first used the type in the United States. In the case of the Pacific, that was the Missouri Pacific Railroad in 1902.
In the Cape of Good Hope in South Africa, the first Pacifics were delivered from Kitson & Company in 1903 and designated the Karoo Class, from the region of the Cape Western System of the Cape Government Railways that they were designed to work in.

Global popularity

The Pacific type was used on mainline railways around the world. The railways of New Zealand and Australia were the first in the world to run large numbers of Pacific locomotives, having introduced types in 1901 and 1902 respectively and operating them until the 1960s.
Image:PRR K5 5698.jpg|thumb|left|Builder's photograph of Altoona-built K5 no. 5698, 1929
During the first half of the 20th century, the Pacific rapidly became the predominant passenger steam power in North America. Between 1902 and 1930, about 6,800 locomotives of the type were built by North American manufacturers for service in the United States and Canada. With exported locomotives included, about 7,300 were built in total. About 45% of these were built by the American Locomotive Company which became the main builder of the type, and 28% by Baldwin. Large numbers were also used in South America, most of which were supplied by manufacturers in the United Kingdom, the United States and Germany.
Africa was the third continent upon which the Pacific was regularly used, following the introduction of the Karoo class on the Cape Government Railways in the Cape of Good Hope in 1903. The earliest African examples were built in the United Kingdom by Kitson & Company.
The earliest examples of the Pacific in Europe were two French prototypes, introduced in 1907 and designed by the Compagnie du chemin de fer de Paris à Orléans to overcome the insufficient power of their. Within a few weeks, these were followed by a German Pacific type that, although already designed in 1905, only entered service in late 1907. The next was a British type, introduced in January 1908. By the outbreak of World War I, the type was being widely used on the railways of Continental Europe.
The Pacific type was introduced into Asia in 1907, the same year that it was first used in Europe. By the 1920s, Pacifics were being used by many railways throughout the Asian continent.
In 1923, the Pacific gave its name to Arthur Honegger's orchestral work, Pacific 231, which successfully reflectively interprets the emotive sounds of a steam locomotive.

Tank locomotives

During the first two decades of the 20th century, the Pacific wheel arrangement enjoyed limited popularity on tank locomotives. On a locomotive, the trailing wheels support the coal bunker rather than an enlarged firebox and such a locomotive is therefore actually a tank engine version of the Ten-wheeler tender locomotive. Indeed, many of the earliest examples were either rebuilt from tender locomotives or shared their basic design.
Around 1920, it became apparent to designers that the wheel arrangement allowed a too limited bunker size for most purposes, with the result that most later designs of large suburban tank classes were of the Hudson or Adriatic wheel arrangement.

Lifespan

The Pacific became the major express passenger locomotive type on many railways throughout the 1920s, 1930s and 1940s. Examples were also built for fast freight and mixed traffic duties. However, due to the increasing weight of trains during the 1940s, larger developments of the type became necessary in the United States and elsewhere. The most notable of these was the Hudson or Baltic type, which had a four-wheel trailing bogie that permitted an even larger firebox, albeit at a loss of some adhesive weight, and the Mountain type which used an extra pair of driving wheels to deliver more tractive effort to the rails. Nevertheless, the Pacific type remained widely used on express passenger trains until the end of steam traction. The last examples were built in the United Kingdom and Japan in the mid-1950s. British Railways introduced its BR Standard Class 6 and BR Standard Class 7 designs in 1951 and 1952, and the final United Kingdom design, the BR Standard Class 8, in 1954.
However, the story of the type did not end in the 1960s. One further mainline example of the LNER Peppercorn Class A1, No. 60163 Tornado, was completed at Darlington by the A1 Steam Locomotive Trust in 2008. Designed to meet modern safety and certification standards, Tornado runs on the United Kingdom's rail network and on mainline-connected heritage railways.

Usage

Argentina

The Vulcan Foundry built twenty Pacific locomotives for the former Buenos Aires Great Southern Railway in 1926, two of which still survive. A further single 12B class locomotive was built in 1930, and the 12K class of twelve Pacific locomotives was built for the BAGSR by Vulcan in 1938.
In 1930, the Central Argentine Railway ordered twenty large three-cylinder PS11 class Pacific locomotives with Caprotti valve gear, which were at the time the most powerful locomotives on the FCCA. In 1939, one of these set up a South American speed record, averaging on the El Cordobes express across the non-stop run from Rosario to Buenos Aires, hauling a 500-ton train and at times attaining a maximum speed of nearly.
The Vulcan Foundry built a further fifty locomotives of a modernised PS12 class version of this design for the nationalised Ferrocarriles Argentinos between 1950 and 1953.

Australia

In Australia, the first known example of the wheel arrangement was the Q class tank locomotive of the gauge Western Australian Government Railways. The six locomotives were introduced in 1896, but four of them were soon converted to a Hudson configuration.
The WAGR was the largest user of Pacific tender types in Australia. In total, the WAGR operated at least 223 locomotives, acquired between 1902 and 1950, making it by far its most numerous wheel arrangement.
  • The first simple expansion tender locomotives in Australia were ordered from British manufacturers for the WAGR. However, due to slow delivery times by the British companies as a result of full order books and their preference for larger orders, twenty compound expansion locomotives were also ordered from Baldwin Locomotive Works in Pennsylvania and placed in service in 1902, designated the Ec class. From 1923, these locomotives, designed for heavy goods and passenger traffic, were converted into light-lines L class engines, but without altering their wheel arrangement.
  • The British-built locomotives, the WAGR E class, were built by Nasmyth, Wilson & Company, North British Locomotive Company and Vulcan Foundry between 1902 and 1912. These were the first of the type to be introduced in quantity, with 65 locomotives in the class.
  • Other Pacific type locomotives operated by the WAGR included the twenty-strong C class, introduced in 1902, which were converted from a to a wheel arrangement from 1909.
  • It was not until the introduction of the WAGR P class in 1924 that Western Australia received what many considered a true Pacific, a large, well balanced locomotive designed primarily for fast passenger traffic. The P class consisted of 25 locomotives, built in 1924 and 1925 by NBL as well as locally at the WAGR's Midland Railway Workshops. The P class engines revolutionised express passenger travel in Western Australia by drastically reducing passenger travel times between destinations.
  • The first batch of ten WAGR Pr River class locomotives, named after prominent rivers in Western Australia and with a boiler pressure rating of compared to the of the P class, were built in 1938 at the Midland Railway Workshops. Between 1941 and 1944, eight of the P class locomotives were also converted to Pr class. One of them was preserved.
  • The WAGR U class of fourteen oil burning locomotives, one of which was preserved, were purchased from NBL in 1946 as surplus war-work engines, following the World War II.
  • The WAGR's final Pacific design was its Pm and Pmr classes of 35 locomotives, introduced in 1950, five of which have been preserved. These locomotives were intended to replace the Pr class, but were quickly relegated to goods workings after proving to be rigid steamers.
The Midland Railway of Western Australia, one of the longest-lived privately owned railways in Australia, followed the WAGR's example by introducing five class C locomotives which were built by Kitson & Company to the Karoo design of the Cape Government Railways as basis.
In the 1920s, heavy Pacific locomotives were introduced by both South Australian Railways and Victorian Railways, in response to increasingly heavy passenger trains and the demand for faster services. Although similar in size, power and top speed, their designs reflected different approaches.
The VR also built a locomotive class, the Dde class that was developed from a successful Dd class design in 1908, intended for outer suburban passenger services in Melbourne.
The gauge Queensland Railways had two Pacific locomotive classes. Between 1926 and 1947, 83 B18¼ class were introduced to haul mail trains. The prototype, built by QR's Ipswich Workshops, began trial runs on 16 July 1926 and was followed by two batches of eight locomotives in 1927 and 1929. The last locomotive of this class was delivered in 1947. In 1950, QR ordered 35 BB18¼ class locomotives from Vulcan Foundry, developed from the successful B18¼ class. Another twenty were built by Walkers Limited of Maryborough between 1955 and 1958. Of these, no. 1089 was the last mainline steam locomotive to be built in Australia.
File:3801 2.JPG|thumb|New South Wales Government Railways 3801
The New South Wales Government Railways introduced its C38 class for express passenger service in 1943. These two-cylinder Pacifics had a free-steaming boiler and were renowned for their performance. Class leader 3801 achieved considerable fame in preservation, with notable feats such as hauling the Western Endeavour, a transcontinental journey from Sydney to Perth in 1970.
The Tasmanian Government Railways owned fourteen Pacific locomotives. Four R class were built for passenger trains by Perry Engineering in Adelaide and ten M class were built by Robert Stephenson & Hawthorns in 1951, and were used on all trains on major lines in northern Tasmania.