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4-6-2 "Pacific" Steam Locomotive
Under the Whyte notation for the classification of steam locomotives, 4-6-2 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 4-6-2 locomotive became almost globally known as a Pacific type.

The introduction of the 4-6-2 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the motive power for express passenger trains throughout much of the early to mid-20th century, before either being superseded by larger types in the late 1940s and 1950s, or replaced by electric or diesel-electric locomotives during the 1950s and 1960s.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 4-4-2 Atlantic type, although its prototype had a direct relationship to the 4-6-0 Ten-wheeler and 2-6-2 Prairie, effectively being a combination of the two types.[4] 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 2-6-2 Prairie, the six driving wheels which allowed for a larger boiler and the application of more tractive effort than the earlier 4-4-2 Atlantic, and the two-wheel trailing truck, first used on the New Zealand 2-6-2 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 4-6-0 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 126 miles per hour (203 kph) has been held by a British Pacific locomotive, the Mallard, since 3 July 1938.

Development
The two earliest 4-6-2 locomotives, both created in the United States of America, were experimental designs which were not perpetuated. In 1887, the Lehigh Valley Railroad experimented with a 4-6-0 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 and St. Paul Railway rebuilt a conventional 4-6-0 with trailing wheels as a means of reducing its axle load.

In 1896, six Q class 4-6-2 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 (NZR) from the Baldwin Locomotive Works of Philadelphia, Pennsylvania. The NZR Chief Mechanical Engineer, A.L. Beattie, ordered thirteen new Q class locomotives with a sufficiently large firebox that would be able to efficiently burn poor grade lignite coal from eastern South Island mines. Even before Baldwin had completed the order from New Zealand, their engineers realized the advantages of the new type and incorporated it into their 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 4-4-2 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 and 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 4-6-2 types in 1901 and 1902 respectively and operating them until the 1960s.

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 (ALCO) 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 and 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 (PO) to overcome the insufficient power of their 4-4-2 Atlantics. 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 the First World War, 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. (231 after the French system of counting axles rather than wheels.)

Tank locomotives
During the first two decades of the 20th century, the Pacific wheel arrangement enjoyed limited popularity on tank locomotives. On a 4-6-2T 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 4-6-0 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 4-6-2T 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 4-6-4T Hudson or 2-6-4T 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 4-6-4 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 4-8-2 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 Standard class 6 and class 7 designs in 1951 and 1952, and the final United Kingdom design, the Standard Class 8, in 1954.

However, the story of the 4-6-2 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.

United States
The Pacific Type was first used in the United States in 1886. This was an unusual double-cab or Mother Hubbard type with an unusually huge firebox, designed to use the waste tailings from anthracite coal mines. While this design did not become popular, the 4-6-2 was rediscovered for the same reason, to improve the 4-6-0 Ten-wheeler with a larger firebox.

With altogether 697 Pacific locomotives, the Pennsylvania Railroad (PRR) was the largest user of the type in the United States. The railroad bought its first experimental K-28 class 4-6-2 from ALCO in 1907. After testing, a further 257 Pacific locomotives in various versions, designated classes K-2, K-2a, K-2b and K-3, were built by the PRR at its Altoona Works and by ALCO and Baldwin between 1910 and 1913.

In 1911, the PRR ordered an experimental K-29 class 4-6-2 from ALCO, with a larger boiler, superheater, mechanical stoker and other innovations. A similar K4s class locomotive was built by the PRR in 1914, but no more were built until 1917. Between 1917 and 1928, the PRR built 349 K-4s locomotives and Baldwin a further 75, bringing the total of the K4s class to 425.

The last PRR Pacific locomotives were two large K-5 class locomotives, built in 1929. No. 5698 was built at the PRR Altoona Works and had Walschaerts valve gear, while no. 5699 was built by Baldwin and had Caprotti valve gear. Although successful, these locomotives were not replicated, since the larger 4-8-2 Mountain types began to be introduced. No. 5698 was dropped from the roster in October 1952 and no. 5699 was retired in September 1953.

The first modern example of the type to be built for duty in the United States, was built for the Missouri Pacific in 1902, but the chief proponent of the type west of the Mississippi River was the Atchison, Topeka and Santa Fe Railway, who began buying the type the next year and ultimately owned 274. The road would have pioneered the type, if not for a belief that a two-wheeled lead truck would be sufficient for high speed passenger service. They began buying 2-6-2 Prairie types in quantity from Baldwin in 1901, with the four cylinder Vauclain compound system, a weight of 190,000 pounds (86,183 kg) and 79 inches (2,007 mm) diameter coupled wheels.

When these proved insufficiently stable for high speed service, the road ordered the 1200 class of 4-6-2 Pacifics, which were two cylinder simplex engines weighing 220,000 pounds (100,000 kg) and fitted with 69 inches (1,753 mm) diameter coupled wheels on unusually long axle centers. Immediately upon their arrival on the property, their drive wheels were swapped with the 79 inches (2,007 mm) diameter drivers off the earlier Prairie types, which became fast freight locomotives. These would wind up in branch line service, where they were very successful and ultimately outlasted the Pacifics.

The Santa Fe ordered additional Pacific types of both four cylinder balanced compound and two cylinder simple types in seven classes through 1914. These gradually increased to 276,500 pounds (125,418 kg) and invariably rode on 73 inches (1,854 mm) drivers. The simple types tended to run conservative pressures at 170 to 175 pounds per square inch (1,170 to 1,210 kpl), while the compounds ran at 220 to 175 pounds per square inch (1,520 to 1,210 kpl). The early examples utilized a firebox grate of 54 square feet (5 sq. m), but the last few classes had larger grates of 57.6 sq. ft. (5.35 sq. m). All of these were considered light Pacifics by the road, and there were a few engines of orphan classes as well. Some of these were scrapped as compounds, but most were rebuilt with two 23½" X 28" (597 x 711 mm) simple cylinders and 220 pounds per square inch (1,500 kpl) operating pressure.

The railroad began scrapping these in 1932, and regretted scrapping those few during the massive traffic of the Second World War. Two were semi-streamlined for a brief period during 1939. They hauled all manner of passenger trains, and saw occasional duty in local freight and helper service. All were out of service by 1955. They initially served on the western portion of the Santa Fe system, west of La Junta, Colorado, where the line traversed the Rocky Mountains and tackled the mountains of California. 4-4-2 Atlantic types were generally used on the Great Plains. Later, as passenger cars grew to 85 feet (26m) in length and gained weight due to all-steel construction, Pacifics would replace the Atlantic types in the east and the western stretches would be served by new 4-8-2 Mountain and 4-8-4 Northern types.

These engines were not dissimilar to the USRA Light Pacifics introduced during the First World War, but differed in certain respects. The Santa Fe, like most large United States railroads, was accustomed to custom-designing their own power and refused to buy USRA designs during the ill-fated nationalization of the United States railroads under Wilson. This era, however, did allow many smaller railroads, who could not afford to custom-design power, to modernize their fleets and it also saw the rise of the USRA Heavy Pacific. The Pennsylvania K-series served as a prototype for these, but they differed in important aspects such as the PRR's Belpaire fireboxes.

The Santa Fe did not buy any USRA Heavy Pacifics, either, but after the war, Baldwin began building the new and even heavier 3400 Class for the road. These were huge at 288,000 to 310,350 pounds (130,635 to 140,772 kg), but were otherwise a conservative design with two simple 25 x 28 cylinders, Walschaerts valve gear, 66.8 square feet (6.2 sq m) of grate and 200 pounds per square inch (1,400 kpl) boilers. Fifty were built by Baldwin through 1924 but, while improvements to the light Pacifics were mostly confined to simplification and other updates were only sporadically applied, all of the 3400s were built or retrofitted with feedwater heaters and all but six were to receive 79 inches (2,007 mm) diameter driving wheels before or during the Second World War. All got a pressure increase to 220 pounds per square inch (1,500 kpl), nine received thermic syphons, and a little experimentation was done with combustion chambers and roller bearings. Weights ultimately reached 312,000 to 326,000 pounds (141,521 to 147,871 kg). These, too, were mostly out of service by 1955. Six Santa Fe Pacific types survive, most of them of the heavy 3400 Class.

Most of the United States railroads which offered passenger service, used Pacific types. Except for the custom design and sheer volume of units produced, the experience of railroads in the eastern and western United States was not dissimilar to that of the Pennsylvania and Santa Fe, respectively. Some roads developed these into the Hudson (or Baltic) type 4-6-4, others preferred the versatility of the 4-8-2 Mountain and 4-8-4 Northern types, and some, like the Santa Fe, bought both. One railroad, the St. Louis-San Francisco or Frisco, actually converted a few existing Pacific types to Hudsons with larger fireboxes in their Springfield shops. The Pacific type, however, was far and away the predominant passenger service steam engine in the United States until the end of steam. Lighter streamlined cars led to a resurgence of the light Pacific, with several railroads applying streamlined shrouds to older engines. The last Pacific built for service in the United States was delivered to the Reading in 1948. Most or all Pacifics were out of regular service by 1960.

One notable 4-6-2, the Soo Line No. 2719 which hauled the last of the Soo Line Railroad’s steam-powered trains in 1959, was preserved and was restored to operating condition for excursions. She is now on display at the Lake Superior Railroad Museum in Duluth, Minnesota.

Canada
The Canadian Pacific Railway (CPR) employed several Pacific classes, beginning with 39 G1 class locomotives, built between 1906 and 1914 by the CPR at its Angus Workshops and by the Montreal Locomotive Works. After 1921, 166 examples of a new G2 class locomotive with a superheater were built by the American Locomotive Company at Schenectady, Angus and Montreal. The last of these remained in service until 1961.

After the First World War, the CPR needed heavier mixed traffic locomotives since steel passenger cars replaced the older wooden ones on its mainlines. This resulted in the introduction in 1919 of 23 G3a class 4-6-2s with 75 inches (1,905 mm) driving wheels, built by Angus for service over flat terrain, and five G4 class locomotives with smaller 70 inches (1,778 mm) drivers, built by Montreal for hilly terrain. A further 152 G3 class locomotives were built in batches between 1926 and 1948. These locomotives were withdrawn from service between 1954 and 1965.

102 examples of the G5 class locomotive were built after 1944. The first two were built by Angus and the rest by Montreal and the Canadian Locomotive Company. They were considered fast, efficient and handsome locomotives and remained in service on many secondary lines of the CPR until the end of steam.

The Reid-Newfoundland Company Limited, which operated the railways in Newfoundland, took delivery of ten Pacific locomotives with 42 inches (1,067 mm) drivers between 1920 and 1929, built by Baldwin, Montreal and ALCO Schenectady. Numbered 190 to 199, they had two 18 by 24 inches (457 by 610 mm) cylinders and weighed 56.3 tons. They all passed to the Government-owned Newfoundland Railway, and then to Canadian National Railways (CN) when Newfoundland joined the Confederation of Canada. CN renumbered them 591 to 599 and classified them as J-8-a (BLW 54398–54401 and 54466–54467 of 1920), J-8-b (BLW 59531 and MLW 67129, both of 1926) and J-8-c (ALCO-Schenectady 67941–67942 of 1929).

MexicoThe Canadian National Railway (CN) sold a 3 ft 6 in (1,067 mm) gauge Pacific locomotive, the former CN No. 591, to Ferrocarriles Nacionales de México (NdeM), where it was numbered 139.

They were the only Pacific type locomotives built to operate on 3 ft 6 in (1,067 mm) gauge in North America. The only surviving Newfoundland steam locomotive, the Newfoundland Railway no. 193, later CN no. 593, is preserved and on display at the Humbermouth Historic Train Site in Newfoundland.

4-6-2 Pacific Overview
Equivalent classifications
UIC class: 2C1 (refined to 2′C′1′ or 2′C1′)
French class: 231
Turkish class: 36
Swiss class: 3/6
Russian class: 2-3-1
First known tank engine version
First use: 1896
Country: Australia
Locomotive: Q class
Railway: Western Australian Government Railways
First known tender engine version
First use: 1887
Country: United States
Railway: Lehigh Valley Railroad
Evolved from: 4-6-0
Benefits: Experimental with Strong's firebox
First known "True type" version
First use: 1901
Country: New Zealand
Locomotive: Q class
Railway: New Zealand Railways Dept.
Designer: A.L. Beattie
Builder: Baldwin Locomotive Works
Evolved from: 4-4-2, 4-6-0 & 2-6-2
Benefits: Wide and deep firebox behind coupled wheels

See also:

Steam Locomotives

Diagram of the 4-6-2 Wheel Arrangement: two small leading wheels, three large driving wheels joined with a coupling rod, and a single small trailing wheel. Front of locomotive at left.

A 4-6-2 Pacific Slideshow.

Baltimore & Ohio 4-6-2 Pacific Type1. Weight - 263,800 lbs, Length - 80 ft., Height of Driving Wheels - 74 inches. 

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Baltimore & Ohio 4-6-2 Pacific Type1. Weight - 263,800 lbs, Length - 80 ft., Height of Driving Wheels - 74 inches.
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