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A Garratt is a type of Steam Locomotive that is Articulated , normally in three parts. Its Boiler is mounted on the center frame, and two Steam Engines are mounted on separate frames, one on each end of the boiler. Because a Garratt locomotive has the Wheel Arrangement of two locomotives back to back, it is typically named "Double ''x''," where ''x'' is a named arrangement. The 4-6-2 arrangement is often called the Pacific, so a 4-6-2+2-6-4 Garratt would be a Double Pacific. DEVELOPMENT The concept for this type of locomotive was developed by Australia n Herbert William Garratt , the New South Wales Railways ' Inspecting Engineer in London , who first applied for a Patent on the idea in 1907 . Its principal advantage is that, because the weight of the engine is spread out over a large area, a very powerful locomotive can be operated on lightweight, narrow gauge track. Its second most obvious advantage is that a very large locomotive can turn in a comparatively sharper curve than a similarly large rigid-frame loco can. A third advantage is that one crew can operate the equivalent of two smaller engines that would require two crews. BEYER-GARRATT Garratt's patent was purchased by the British firm of Beyer-Peacock who developed it, marketed and sold it extensively, as well as licensing the design to other builders; for this reason, the name Beyer-Garratt is regularly used in many countries. The majority of Garratts were built in the United Kingdom. Just under two-thirds (1023 of 1651) were built by Beyer-Peacock, while the rest were constructed by a variety of licensees, including builders in Britain, France, Spain, Germany, Italy, Belgium, South Africa, Brazil, and Australia. The Garratt was most widely used throughout Africa, the largest class used therein (and indeed the largest and most powerful in the Southern Hemisphere) being the South African Railways' eight GL class locomotives. Garratts also appeared in Asia, Australia, South America, a few European countries, and the British Isles. British usage was slight, despite Beyer-Peacock's location. There were no Garratts in North America; all articulateds there were of the Mallet Style (many were simple rather than compound, and thus not "true" Mallets). In the UK a single large Garratt (2-8-0+0-8-2, number 2395/69999 LNER Class U1 ) was built in 1925 for banking heavy coal trains on the Woodhead route. A class of 33 2-6-0+0-6-2 Locomotives were built for the LMS and several 0-4-0+0-4-0s were built for Industrial Use , one of which is preserved. The New South Wales Government Railways introduced the 4-8-4+4-8-4 AD60 Garratt in 1952, built by Beyer Peacock. The AD60s weighed 265 tonnes, with only a 16 tonne axle loading. They had a tractive effort of 265 kN, and were the most powerful locomotives in the southern hemisphere at that time. On one occasion, a 1220-tonne, double-headed, diesel freight failed on a 1 in 66 grade. An AD60 was used to clear the dead train from the section. It pulled the entire load (now 1450 tonnes) up the grade without any wheel slip. DISADVANTAGES The major disadvantage of a Garratt (shared with all Tank Engine s) is that the tractive weight reduces as the water is used from the tanks. This can lead to problems with slipping and is one reason why some of the locomotives used in Africa ran with an additional tank wagon containing water (this also reduced the axle load). Another disadvantage, when compared with two separate locomotives, is that both power units are controlled by one regulator, thus if one power unit slipped the steam to both was reduced as the driver tried to control the slipping. The Garratt has a potential safety problem should it have to operate through tunnels, especially those of a narrow profile. Should a Garratt hauled train stall in a tunnel the crew may become trapped, since there is no route forward or backwards past the hot cylinders. A normal engine has hot cylinders at only one end, and there is always an escape route at the other end. In this way a Garratt crew is in exactly the same position as the crew of a pilot engine coupled to the front of a train to assist the train engine up a bank, and it is notable that such double-headed steam working in many tunnels was banned for precisely this reason of safety. In Western Australia the use of the Australian Standard Garratt {Link without Title} of the 1940's through the state's only tunnel at Swan View caused serious problems as stated above, resulting in deaths and in a Royal Commission. Designed and built in Victoria in 1943, the ASG Garratt was also used in Queensland . However, by September 1945 the class had been withdrawn, as it had a tendency to derail due to its unflanged leading coupled wheels. Queensland Rail later introduced Beyer Garratts and these were considerably more successful than the ASG. In Tasmania the Emu Bay Railway utilised ASG engines with success and few problems at the same time. Tasmania by coincidence was also the location of the railway to run the very first Garratt locomotives the TGR K Class (K1 and K2). SOURCES
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