Soo Line 4-6-2 No. 2719 steams from Duluth to Two Harbors, Minnesota.

Pete Markham, CC BY-SA 2.0 <>, via Wikimedia Commons



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 after a New Zealand locomotive that was shipped across the Pacific Ocean.


A diagram of the 4-6-2 wheel arrangement. Front of locomotive at left.

(Gwernol, Public domain, via Wikimedia Commons)



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the motive power for express passenger trains throughout much of the early to mid-20th century, before either being superseded by larger types in the late 1940s and 1950s, or replaced by electric or diesel-electric locomotives during the 1950s and 1960s.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. 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 km per hour) has been held by a British Pacific locomotive, the Mallard, since 3 July 1938.



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 & St Paul Railway rebuilt a conventional 4-6-0 with trailing wheels as a means of reducing its axle load.


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.


Missouri Pacific No. 6001 4-6-2 Steam Locomotive. ca. 1940s. Photo by R. J. Foster.

(R.J. Foster, via W. Lenheim Collection)



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.


PRR K4 Pacific No. 1361 travels along the New York and Long Branch Railroad between Amboy, NJ and the Northern Jersey shore.

This locomotive was later moved to static display at Horseshoe Curve. Photo by Jim Kelly. Click to enlarge.

(Audio Visual Designs, Earlton, NY, via W. Lenheim Collection)

United States

The 4-6-2 wheel arrangement was first used in the United States in 1886. This was an unusual double-cab or Mother Hubbard type with an unusually large 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 K4 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 class K5 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 (86,000 kg) and 79-inch-diameter (2,007 mm) 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-inch-diameter (1,753 mm) coupled wheels on unusually long axle centers. Immediately upon their arrival on the property, their drive wheels were swapped with the 79-inch-diameter (2,007 mm) 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 (125,400 kg) and invariably rode on 73-inch (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 kPa), while the compounds ran at 220 to 175 pounds per square inch (1,520 to 1,210 kPa). The early examples used a firebox grate of 54 square feet (5.0 m2), but the last few classes had larger grates of 57.6 square feet (5.35 m2). 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 23+1⁄2-by-28-inch (597 mm × 711 mm) simple cylinders and 220-pound-per-square-inch (1,500 kPa) operating pressure.

The railroad began scrapping these in 1932, but regretted it during the massive traffic of World War II. Two were semi-streamlined for a brief period during 1939. They hauled varied 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. 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 World War I, 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 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 (130,630 to 140,770 kg), but were otherwise a conservative design with two simple 25-by-28-inch (635 mm × 711 mm) cylinders, Walschaerts valve gear, 66.8 square feet (6.21 m2) of grate and 200-pound-per-square-inch (1,400 kPa) 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 World War II. All got a pressure increase to 220 pounds per square inch (1,500 kPa), 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 (142,000 to 148,000 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 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. It is now on display at the Lake Superior Railroad Museum in Duluth, Minnesota.


Canadian Pacific 1201, a Class G5-a 4-6-2 at the CP's Montreal Shops. Click to enlarge. (Audio Visual Designs, Earlton, NY, via W. Lenheim Collection)


Canadian Pacific Railway G3c class 4-6-2 locomotive No. 2317. Click to enlarge. (Photo part of the Historical American Engineering Record by the US Parks Service, public domain)



Canadian Pacific (CP) employed several Pacific classes, beginning with 39 G1 class locomotives, built between 1906 and 1914 by the CP at its Angus Shops 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 World War I, the CP 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-inch (1,900 mm) driving wheels, built by Angus for service over flat terrain, and five G4 class locomotives with smaller 70-inch (1,780 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 CP until the end of steam.

The Reid-Newfoundland Company Limited, which operated the railways in Newfoundland, took delivery of ten Pacific locomotives with 42-inch (1,070 mm) drivers between 1920 and 1929, built by Baldwin, Montreal and ALCO Schenectady. Numbered 190 to 199, they had two 18-by-24-inch (460 mm × 610 mm) cylinders and weighed 56.3 tons. They all passed to the Government-owned Newfoundland Railway, and then to Canadian National (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).

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. (Also see Mexico - below)



The Canadian National (CN) sold a 3 ft 6 in (1,067 mm) gauge Pacific locomotive, the former CN no. 591, to Ferrocarriles Nacionales de México, where it was numbered 139. (Also see Canada - above)


PRR K4 No. 3768 in streamlined casing designed by Raymond Loewy, 1936. Click to enlarge.

(, See page for author, Public domain, via Wikimedia Commons)



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 of America
Railway: Lehigh Valley Railroad
Evolved from: 4-6-0
Evolved to: 4-6-4
Benefits: Larger firebox than the 4-6-0
Drawbacks: Required piloting (also known as double heading) when train lengths increased
First known "True type" version
First use: 1901
Country: New Zealand
Locomotive: Q class
Railway: New Zealand Railways Department
Designer: Alfred Beattie
Builder: Baldwin Locomotive Works
Evolved from: 4-4-2, 4-6-0 & 2-6-2
Evolved to: 4-6-4, 4-8-2
Benefits: Wide and deep firebox behind coupled wheels