Although surpassed in speed, power, and size by other locomotives, it is recognised as the most mass-manufactured locomotive in Japanese rail history. A total of 174 units are preserved in Japan, including five operational examples. An additional 13 are preserved in Russia and Taiwan, bringing the total number of preserved units to 187.
The classification consists of a "D" for the four sets of driving wheels and the class number 51 fortender locomotives that the numbers 50 through 99 were assigned to under the1928 locomotive classification rule.
Following the Great Depression of 1929, Japan's rail freight demand declined. This delayed plans for the development and production of new freight locomotives, which were set to become successors to the Class D50. By the mid-1930s, economic recovery led to increased transport demand. Although electrification was in progress, steam locomotives remained the backbone of railway operations.[2] The result was the D51, a mechanically simple, versatile and fuel-efficient freight locomotive. The D51's chief designer,Hideo Shima, later described it as one of his most satisfying designs.[3]
The D51 was based on the earlier Class D50, with which it shares many design fundamentals, such as a parallel boiler and a 1D1 (2-8-2) wheel arrangement. Notable changes from the Class D50 included:[3][4]
Boiler pressure and tractive effort: The boiler pressure was initially raised from the 13 to 14 kg/cm2 (1,300 to 1,400 kPa; 180 to 200 psi) and later to 15 kg/cm2 (1,500 kPa; 210 psi) in wartime models. Despite a smaller cylinder bore, these changes enhanced tractive effort.
Weight reduction and adaptability: Electric welding replaced traditional riveting, which resulted in reduction of axle load and overall weight. The maximum axle load was reduced from 14.99 to 14.30 t (16.52 to 15.76 short tons; 14.75 to 14.07 long tons), which allowed operation on lighter secondary lines previously inaccessible to the D50.
Length reduction: The D51 was 571 mm (1 ft 10.5 in) shorter than the D50 to fit on standard 60 ft (18 m) branch line turntables, thus broadening its operational range.
Wheel design: The D51 adoptedboxpok wheels, which distributed force more evenly across the wheel circumference.
Otherissues identified in the D50, such as derailments during reverse operations, were also addressed in the D51's design.[5]
Early models are noted for their semi-streamlined appearance, with the feedwater heater positioned between the sand dome and chimney and enclosed in a long casing. This design earned the nickname "Slug" (ナメクジ,Namekuji). Two locomotives, D51 22 and 23, had an extended casing reaching the cab, referred to as "Super Slug" or "Big Slug." These were later modified to match other units because the extended casings required extra maintenance. To balance the shortened overall length compared to the D50, the cab was made smaller. While this reduced weight, it also made the cab more confined, which some crews found uncomfortable. The weight of the first driving axle was lighter than the others, which caused wheel slip under heavy loads. The air-powered reverser was also found to be difficult to use due to its sensitivity. Later modifications, such as adding weight to the front deck, improved traction.[3][6]
The feedwater heater was relocated to a crosswise position in front of the chimney, and the suspension system was adjusted to improve axle weight distribution. The air-powered reverser was replaced with a manual version for better control. From D51 101 onwards, this updated design became standard, and most of the class was built to this specification. Wartime versions of these later units used simplified materials and construction methods, including wooden components and the omission of some design details to conserve resources.[3][6]
D51 1105, built in 1944, with a boxy wartime-produced casing
Wartime models were built with substitute materials and reduced decorative elements. The dome was reshaped to a flat design, and wooden parts were used for components such as the smoke deflectors. Some tenders adopted a simpler 'ship-bottom' design. To increase hauling capacity, boiler pressure and axle load were raised, but the use of substandard materials and simpler welding caused reliability issues. This resulted in several boiler explosions, notably with D51 1140. After the war, many of these units were retrofitted with standard parts to improve safety and reliability.[3]
The D51 was widely used across Japan and was a common sight throughout the country. Primarily a freight locomotive, the D51 was occasionally assigned to passenger services, particularly on steeply graded lines such as theChūō Main Line and the mountainous section of theHakodate Main Line (Oshamambe–Otaru). On some flatter lines, such as theUetsu Main Line, it also hauled passenger trains. Notable examples of its service include locomotives assigned to royal trains (onEmperor Shōwa's visit toShimane in 1971)[7] and units used at yards on main lines, such as Shintsurumi andSuita, for hump shunting.
A D51-hauled freight train at Jōmon Pass on theSekihoku Line
From the 1960s, D51 locomotives began to be retired as electrification and dieselisation progressed. However, many remained in service until the final years of steam operation in Japan. During the 1960s and 1970s, they were particularly prominent on steeply graded lines such as theSekihoku Main Line,Tōhoku Main Line,Ōu Main Line, andHakubi Line, often operating in double- or triple-headed configurations. These operations attracted railway enthusiasts, photographers, and media attention during the 'SL boom' period. While its ubiquity made the D51 an iconic machine, some enthusiasts seeking rarer locomotive classes expressed disappointment when encountering D51s.
The first D51s in Hokkaidō were assigned to theOtaru-Chikkō andIwamizawa depots, with units such as D51 6 at Otaru-Chikkō and Nos. 48 and 126 at Iwamizawa. On the Nemuro Main Line, D51 locomotives assigned to the Shintoku depot were used on the oldKarikachi Pass section without smoke deflectors and with limited installations of oil-fired systems.[8] D51 241 at the Oiwake depot hauled JNR' final steam-hauled train on 24 December 1975, on the Yūbari Line.[9]
D51s were extensively used on major lines such as theTōhoku Main Line,Ōu Main Line,Jōban Line, andUetsu Main Line. On the Tōhoku Main Line, triple-heading operations were notable at Jūsanbongi Pass inIchinohe, Iwate, with locomotives from Morioka, Ichinohe, and Shiriuchi depots. Similarly, triple-heading was common at Yatate Pass on the Ōu Main Line until 1971, by locomotives from depots such as Aomori, Hirosaki, and Akita.[10]
In theKantō region, D51s were assigned to depots includingTakasaki,Utsunomiya,Mito,Ōmiya,Hachiōji,Tabata, Shintsurumi, andShinkoiwa. At Mito, some units were fitted with automatic coal-feeding equipment in 1957 to accommodate the low-calorific coal from theJōban coalfield. The electrification of lines such as theSōbu Main Line andTakashima Line in 1970 led to the withdrawal of D51s from depots such as Shintsurumi.[11] D51 791 was used for farewell passenger services in October 1970, which operated between Tokyo and Yokohama.[12]
D51s were assigned to theHokuriku Main Line, including depots atTsuruga,Fukui, andKanazawa, where they handled freight operations over steep passes such as the Yanagase and Yamano Tunnels. The electrification of the Hokuriku Main Line was completed by 1964, but some D51s remained in use for local and yard duties until 1971. On theTakayama Main Line, they replacedC58s and9600s from 1959 until dieselisation in 1969.[13]
D51s operated on lines such as theSanyō Main Line,Hakubi Line, andMine Line. On the Hakubi Line, D51s based atNiimi depot were used for triple-headed limestone freight trains, which attracted significant attention from railway enthusiasts.[15]
Thirteen D51s were assigned to the Dosan Line, based at the Kōchi depot. These locomotives were eventually replaced byDF50 diesel locomotives by 1960.[16]
In Kyūshū, D51s were assigned to depots includingMojikō,Tosu, andKumamoto. On the steeply gradedHisatsu Line, D51s replaced older locomotives such as theClass 4110 for services over the Yatake Pass. These locomotives were equipped with smoke collectors and oil-firing systems.[17]
According to the a journal published in 1956, ten locomotives were built byNippon Sharyo for theManila Railroad Company. These entered service in 1951. Numbered the 300 class, they were named after thecog locomotive class built in the 1910s for the Manila Railway. These locomotives differed from the rest of the D51 builds through the lack ofsmoke deflectors.[20]
The locomotives had a short service life in thePhilippines as Manila Railroad ordered thedieselization of its entire network, having all steam locomotives retired by 1956.[21]
From 1936 to 1944, Kawasaki,Kisha Seizō and Hitachi had built 32 D51s for Imperial Taiwan Railway. AfterWorld War II, they were taken over byTaiwan Railways Administration, and were classifiedDT650. In 1951, Kisha Seizō built three DT650s and Mitsubishi Heavy Industries built two DT650s for Taiwan Railways Administration.[22]
D51 146: Operated on theMooka Railway, runs on compressed air. (Semi-operational)
D51 200: Preserved in operational condition byJR West at theKyoto Railway Museum, and was operating on Yamaguchi Line from November 2017 to May 2022, when a crack in one of its bogies was discovered. Restoration work and trial runs are continuing at the Railway Museum. Re-entered service in 2024.
D51 320: Operated at a railway museum inAbira, Hokkaido on compressed air. (Semi-operational)
D51 498: Restored byJR East, based at Takasaki Rolling Stock Center, and pulls special-event trains on JR East lines.[24][25]
D51 827: Operated at Aridagawa Railway Park inWakayama Prefecture, runs on compressed air.
Built in 1938 at the JNRHamamatsu Works, locomotive number D51 200 has been overhauled and restored to operational condition for use asSL Yamaguchi andSL Kitabiwako starting in 2017.[26]
Hiro, a character based on the Class D51 and voiced byTogo Igawa, appears in the 2009 animated filmThomas & Friends: Hero of The Rails and thereafter became one of the show's mainstays and the first character introduced after the change to CGI.
In the animeHikarian the characterDozilas is based on the D51 498.
TheGobots akaMachine Robo character Loco (Steam Robo) has a vehicle mode based on the D-51 steam train and has the number D-5147 on him.
It also appeared in Initial D Stage 3 at Shibukawa Station as the actual engine.
In the Netflix anime adaptation of Junji Ito Maniac, the second episode portrays a D51 1611 (a numeration that not exists in the reality, likely a reference of the last unit built, the D51 1161.
^abcdefgSasada, Masahiro (September 2012).国鉄&JR保存車大全 [JNR & JR Preserved Rolling Stock Complete Guide]. Tokyo, Japan: Ikaros Publications Ltd. pp. 133–135.ISBN978-4863206175.
^JR全車輌ハンドブック2009 [JR Rolling Stock Handbook 2009]. Japan: Neko Publishing. 2009.ISBN978-4-7770-0836-0.
^JR西日本、D51形200号機が本線運転復活へ [JR West to restore D51 20 to mainline operational condition].Mynavi News (in Japanese). Japan: Mynavi Corporation. 18 October 2014. Retrieved21 October 2014.
^津山市 来月からD51の展示始まる [D51 to go on display at Tsuyama from next month].RSK News (in Japanese). Japan: Sanyo Broadcasting Co., Ltd. 26 February 2015. Archived fromthe original on 26 February 2015. Retrieved31 October 2015.
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