2-6-4

Under the Whyte notation for the classification of steam locomotives, a 2-6-4 locomotive has two leading wheels, six coupled driving wheels and four trailing wheels.

Overview
With only a few known exceptions, the Adriatic wheel arrangement was usually used on tank locomotives, for which various suffixes to indicate the type of tank would be added to the wheel arrangement, for example 2-6-4T for an engine with side-tanks.

Tender locomotives
The earliest known example was the South African Class 6Z, designed by Cape Government Railways (CGR) Chief Locomotive Superintendent Hazlitt Beatty in 1901. The first engines of the class were modified 2-6-2 Prairie locomotives which were equipped with two-axle trailing bogies. In 1902, more were placed in service, but built with the 2-6-4 wheel arrangement. The latter were the first known tender locomotives in the world to be built with this wheel arrangement.

Tank locomotives
Tank engines with the 2-6-4T wheel arrangement were produced for many different railway systems worldwide and were mainly used for freight and suburban passenger working. They have been less successful on express passenger trains. The earliest known example also originated in South Africa, the Pretoria-Pietersburg Railway's 55 Tonner of 1898.

Austria
Two Austrian express tender locomotive types were of this wheel arrangement, the of 1908 and  of 1911 designed by Karl Gölsdorf. The type therefore became known as the Adriatic arrangement, named for the Adriatic Sea which bordered Austria-Hungary.

Brazil
Preserved meter gauge locomotive Dona Joaninha, built to haul sugar cane in Brazil, uses the 2-6-4 wheel arrangement.

Finland
Finland had three locomotive classes with a 2-6-4T wheel arrangement, the Classes Vk1, Vk2 and Vk3. All three classes were nicknamed Iita.

The Class Vk1, numbered 301 to 305, were delivered in 1900 from Baldwin Locomotive Works of Philadelphia, Pennsylvania. The Class Vk1 Iita were also nicknamed Amerikan because they were built in the United States.

The Class Vk2 were numbered 454 to 455.

The Finnish Steam Locomotive Class Vk3 were numbered 456 and 487 to 492. They were built in 1915 by Tampella, a Finnish heavy industry manufacturer, and were used for local passenger duties. One of them, Vk3 No 489, is preserved at the Finnish Railway Museum.

Germany
Two Deutsche Bundesbahn (DB) 2-6-4T Class 66 locomotives, designed for fast goods train and passenger train service, were built in 1955 as part of the DB's Neubaulok construction programme. They were both withdrawn from service in 1968. One, DB 66 002, has been preserved at the Bochum-Dahlhausen Railway Museum. For the private TAG, which operated the railway line from Schaftlach to Tegernsee, the 2-6-4T steam locomotive TAG 8 was built in 1942, using wheels from DRG Class E 79. It was in service until 1970 and has been preserved in the Bavarian Railway Museum.

Sweden
SJ type SB with driving wheels 1.75 m diameter for passenger use, built Motala 1917, one preserved.

SJ type J with driving wheels 1.3 m diameter for mixed traffic use, built Atlas, Motala and Nohab, 1914–1918. Four preserved.

Switzerland
were built by SLM in 1913 for use on the Berne-Neuenberg railway. They were still in use at Basel in 1956. Of slightly British appearance, at that time they were used to transfer stock between the French and Swiss systems, the former having not yet been electrified.

New Zealand
The Ferrymead Railway in Christchurch, New Zealand has a gauge 2-6-4T locomotive that was in regular operation until taken off-line for boiler repairs around 2009. It was built by Baldwin Locomotive Works in 1901.

The Wf class of the New Zealand Government Railways was built between 1903 and 1928 and was a general purpose tank design. It was used all over New Zealand and identical locomotives were also in service as the Ds class of the Tasmanian Government Railways. Three Wf class locomotives survived in preservation:
 * Wf386, one of the engines used on the first Wellington to Auckland train, is preserved at Paekakariki.
 * Wf392 was sold to the Tasmanian Government Railways and used as a Ds class locomotive. Its boiler is on display at Don River Railway
 * Wf393 is preserved at Ferrymead Railway.
 * Wf403 is preserved at Nelson Railway Society.

South Africa
Between 1898 and 1900, the Pretoria-Pietersburg Railway (PPR) placed six 2-6-4T locomotives in service, built by Beyer, Peacock & Company. During the Second Boer War, they were first taken over by the Nederlandsche-Zuid-Afrikaansche Spoorwegmaatschappij (NZASM) and then by the Imperial Military Railways (IMR). After the war, they were taken onto the roster of the Central South African Railways (CSAR) and in 1912 they were assimilated into the South African Railways (SAR) as Class D.

In 1901, the CGR placed four 6th Class 2-6-2 Prairie tender steam locomotives in service, designed at the Salt River works of the CGR and built by Neilson, Reid & Company. During trials, it was found that they were inclined to be unsteady at speed and the locomotive design was therefore modified to a 2-6-4 wheel arrangement by replacing the trailing carrier wheels with a two-axle bogie. Another four locomotives incorporating this modification were ordered later in 1901 and delivered in 1902, the first tender locomotives in the world to be built with this wheel arrangement. The change in design resulted in a marked improvement in the locomotive's stability at speed and the first four locomotives were therefore also modified accordingly. In 1912, when they were assimilated into the SAR, these eight locomotives were all designated Class 6Z.

In 1902, the CGR placed two Type A Adriatic narrow gauge locomotive in construction service on the Avontuur branch, which was being built out of Port Elizabeth through the Langkloof. They were built by Manning Wardle and, at a width of 7 ft, they were the widest locomotives to see service on any of the 2 ft narrow gauge lines in South Africa.

United Kingdom
The first British examples of the 2-6-4T wheel arrangement were two locomotives built for the narrow-gauge Leek & Manifold Valley Light Railway in 1904. The first examples were the class 1B of the Great Central Railway, built in 1914.

Richard Maunsell of the South Eastern and Chatham Railway (SECR) designed the K class in 1914 for express passenger duties, and in 1925 introduced a three-cylinder variant K1 class for the Southern Railway. These locomotives proved to be unsteady at speed and, following the Sevenoaks railway accident in 1927, they were rebuilt as 2-6-0 tender locomotives. Maunsell did however use the type more successfully for his W class freight locomotives of 1930.

Between 1927 and 1947, the London, Midland and Scottish Railway (LMS) built nearly five hundred 2-6-4Ts for suburban passenger work to four similar designs (see LMS/BR Class 4 2-6-4T locomotives). The last of these, the LMS Fairburn, continued to be built by British Railways (BR) until 1951. Fairburn's LMS design also formed the basis of the very successful BR Standard Class 4, which continued to be produced until 1957.

Between 1946 and 1950, George Ivatt of the LMS also built eighteen examples of a very similar design at Derby Works, for use in Northern Ireland. These later became the Northern Counties Committee WT class.

A prototype of the London and North Eastern Railway (LNER) L1 class, designed by Edward Thompson, was built in 1945. The remaining 99 members of the class were built under British Railways administration during the period from 1948 to 1950.

United States
The Reading Company used 2-6-4T Adriatics for commuter service.

Model railroading
The Lionel Corporation used the 2-6-4 wheel arrangement in many of its model steam locomotives, including the 2037 used in the infamous pastel-coloured Girls' Train. Their 2-6-4 model was based on the Pennsylvania Railroad’s K4 class pacific, even though this was a 4-6-2 rather than a 2-6-4.