Garratt
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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 4-6-4 arrangement is often called the Hudson, so a 4-6-4+4-6-4 Garratt would be a Double Hudson.
The concept for this type of locomotive was developed by Australian 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 set of crew operate the equivalent of two smaller engines that would require two crew.
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 sometimes used. 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, but also appeared in Asia, Australia, South America, a few European countries, and the British Isles. British usage was slight, despite Beyer-Peacock's location.
A unique large Garratt machine (2-8-0+0-8-2, number 2395/69999) was built in 1925 for banking heavy coal trains on the Woodhead route. With the electrification of that route in the early 50s, the machine became largely redundant, and was tried as an alternative banker on the Lickey Incline. It was not considered a success, however.
Disadvantage
The Garratt has a safety problem should it have to operate through tunnels, especially narrow profiles.
Should a Garratt hauled train stall in a tunnel, the crew can get trapped, since there is no way out 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.
Sources
- Garratt Locomotives of the World, A.E. Durrant
- A Complete List of All Garratt Locomotives (http://users.powernet.co.uk/hamilton/), Gavin Hamilton
- K1 The World's First Garratt (http://www.bangor.ac.uk/ml/whr/k1.htm), Ben Fisher
- The Garratt Locomotive (http://www.austehc.unimelb.edu.au/tia/460.html), Technology in Australia, 1788-1988