Under the Whyte notation for the classification of steam locomotives by wheel arrangement, 2-6-6-0 is a locomotive with one pair of unpowered leading wheels, followed by two sets of three pairs of powered driving wheels and no trailing wheels. The wheel arrangement was principally used on Mallet-type articulated locomotives. Some tank locomotive examples were also built, for which various suffixes to indicate the type of tank would be added to the wheel arrangement, for example 2-6-6-0T for an engine with side-tanks.

Overview

The 2-6-6-0 wheel arrangement was most often used for articulated compound steam Mallet locomotives. In a compound Mallet, the rear set of coupled wheels are driven by the smaller high pressure cylinders, from which spent steam is then fed to the larger low pressure cylinders that drive the front set of coupled wheels.

Usage

Indonesia

The Java Staatsspoorwegen (SS/JSS) operated its first of 2-6-6-0Ts in the 1904. The first batch was delivered between 1904 and 1909 from Schwartzkopff and Hartmann. The second batch, delivered between 1910 and 1911, was built by Werkspoor. The front water tanks of the first batch are square, those on the second batch are sloped. These were successor of the first Mallets the SS Class 500s (DKA BB10s), which were delivered in 1900 and worked on the gauge heavy mountain lines of West Java. Just after arrived on Java, these engines classified as SS Class 312–387, but later renumbered as SS Class 520 (521–543) and worked for mixed passenger and freight trains on Purwakarta–Bandung and Cicalengka–Banjar lines. After Japanese occupation and Indonesian Independence, these locomotives were renumbered to CC10 and used by Djawatan Kereta Api (DKA) or The Department of Railway of the Republic of Indonesia until the era of Perusahaan Jawatan Kereta Api (PJKA). By the end of the service, the CC10s were allocated in Rangkasbitung and Cibatu depots. All of CC10s were withdrawn from active service in 1980s and scrapped. In 1927–1928, the Java Staatsspoorwegen imported 16 2-6-6-0 Mallet locomotives from the Swiss Locomotive and Machine Works and 14 from Werkspoor, N.V. for the mountain lines in West Java. These were a compact version of the Bull Moose SS Class 1250 (DKA DD52). They worked on Cibatu–Garut–Cikajang and Purwakarta–Padalarang lines. The experience gained by the JSS modifying earlier Mallets helped them design these locomotives to use standard parts that could be exchanged with other SS locomotives. These locomotives also known as De Bergkoningin or The Queen of the Mountain. They were known as Si Gombar by locals due to the ability on the mountain lines and its huge size. The Class 1600 proved more powerful than other SS locomotives, and able to haul trains weighing 1,300 tons at a speed of 55 kph, and being able to negotiate tight curves. These locomotives were used throughout Java. During the Japanese occupation of the Dutch East Indies in 1942, all of JSS locomotives were renumbered based on the Japanese numberings. The Class 1600s were renumbered CC50. This numbering system remained in use after the Independence of Indonesia by Djawatan Kereta Api (DKA) - the Department of Railways of the Republic of Indonesia until the era of Perusahaan Jawatan Kereta Api (PJKA) or Indonesian Railway Bureau Company. By 1971, these locomotives were located at Cibatu, Banjar, Purwakarta, Purwokerto, Ambarawa, Madiun and Sidotopo (Surabaya) locomotive depots.

Preservation

The steam era in Indonesia ended in the early 1980s. The remaining locomotives were dumped in station sheds. The last CC50 retired in 1984 after the closure of Cibatu-Garut line. In 1981, CC5022 from Purwokerto depot was donated by PJKA to the Railway Museum (Netherlands), as a symbol of friendship between Indonesia and the Netherlands. This locomotive was returned to its original colonial number 1622. In 2023, only three CC50s remain: 01 and 22 built by Werkspoor and SLM 29. CC5001 is preserved at the Transportation Museum of Taman Mini Indonesia Indah, CC5022 at Netherlands Railway Museum and CC5029 at Ambarawa Railway Museum.

New Zealand

The sole NZR E class locomotive of 1906 was the only 2-6-6-0T locomotive ever built for and used by the New Zealand Railways Department. It was built at the Petone Workshops in Wellington and was designed for use on the world famous Rimutaka Incline. Numbered 66, making it E 66, it spent the first part of its working life in the Wellington region hauling trains up and down the Rimutaka Incline. It was eventually transferred to the Wellington-Johnsonville section for banking duties, even though it was not designed for that type of work. In 1917, E 66 was withdrawn from service and scrapped. It did not survive long enough for preservation.

South Africa

The South African Railways (SAR) operated 57 Mallet locomotives with this wheel arrangement, spread over six classes, all of them.

United States

At least two American railroads used 2-6-6-0 Mallet locomotives. One was the Denver, Northwestern and Pacific Railway, which later became the Denver and Salt Lake Railroad and eventually the Denver and Salt Lake Railway. Towards the end of their service life, after the acquisition of the Denver and Salt Lake, these locomotives were used by the Denver and Rio Grande Western Railroad (D&RGW). The locomotives were initially used across the Rollins Pass and later on the Moffat Tunnel route of the Denver and Salt Lake. None were preserved; they were all scrapped by the D&RGW between 1947 and 1952. Another was the Virginian Railway, whose Class AA 2-6-6-0 is depicted.