Garratt locomotive

thumb|300px|Characteristics of the Garratt articulated locomotive

thumb|300px|The Garratt with the highest tractive effort in the world was the East African Railways 59 class. Its tractive effort of 83,350 pounds was twice that of a [[BR Standard Class 9F|British Railways class 9F 2-10-0]]

A Garratt locomotive is a type of three-part articulated steam locomotive invented by the engineer Herbert William Garratt. Its boiler, firebox, and cab are mounted on a centre frame or "bridge". The two other parts, one at each end, have a pivot to support the central frame; they consist of a steam engine unit – with driving wheels, trailing wheels, valve gear, and cylinders, and above it, fuel and/or water storage.

Articulation permits locomotives to negotiate curves that might restrict large rigid-framed locomotives. The design also provides more driving wheels per unit of locomotive weight, permitting operation on lightly engineered track. Garratt locomotives produced as much as twice the tractive effort of the largest conventional locomotives of railways that introduced them, reducing the need for multiple locomotives and crews.

Advantages of the Garratt design

The principal benefit of the Garratt design is that the boiler and firebox unit are slung between the two engine units. A further advantage is that the firebox and ashpan are not restricted in dimensions by running gear; the ashpan can have much larger capacity than on a normal locomotive, allowing longer continuous runs without needing to stop and empty the ashpan to clear combustion products from the grate. A large firebox and its unrestricted air supply also allowed the Garratt to operate with poor-quality fuel without reducing steaming capacity.

Garratts ran equally well in either direction, negating the need for turntables. Often they ran with the cab leading the boiler (sometimes called [fuel] bunker leading), especially on routes with tunnels. the Garratt type holds several advantages over the Mallet type:<blockquote>This [the Mallet] was so designed to provide a very large engine unit, to be managed by a single crew, but to spread the dead weight over many axles and thus avoid excessive loads on the track and under-line bridges, and at the same time retain flexibility of wheelbase to facilitate operation on severely curved sections of line. The Mallet, having driving-wheel units beneath the boiler, retains the limitation in dimensions inherent in the orthodox type of steam locomotive, while the immense length of some of these machines is itself a handicap. Except with oil firing, ... one cannot put the cab in front. H.W. Garratt ... patented ... the idea of having a single large boiler slung on a cradle carried on two entirely separate engine units. ... The boiler could be developed to ideal proportions, unfettered by any wheels beneath it. It could be kept short, and of large diameter, which is the best possible form for securing a high rate of evaporation.</blockquote>

Although at the end of the steam locomotive era, most conventional steam locomotives had reached their maximum in "critical dimensions", the Garratt still had potential for further development, with larger driving wheels, larger boilers, and greater output still achievable.

Competitors, look-alikes, and variations on the theme

thumb|right|South African Union Garratt

The Garratt was not alone in the field of articulated locomotives; most notably, articulated locomotives in the United States based on the Mallet design achieved power outputs far exceeding those of Garratts. Away from North America were the Fairlie and Meyer articulated types. Further, similar designs to the Garratt were the Union Garratt, Modified Fairlie, and Golwé. Of these, the closest was the Union Garratt, a type originally prompted by the perceived necessity for a rigid connection between a bunker or tender and a firebox fed by a mechanical stoker. They were, in effect, a hybrid Fairlie and Garratt with the rear bunker attached to the frame instead of being carried on the rear bogie. The class GH and class U Union Garratts of the South African Railways were examples.

Garratt development

thumb|right|A builder's photo of the K1 locomotive ([[Tasmanian Government Railways K class), the first Garratt locomotive]]

thumb|right|Works drawing of K1

Herbert William Garratt, a British locomotive engineer, invented the articulated locomotive concept that bore his name, for which he was granted a patent (no. 12079). At the time, he was the New South Wales Railways' inspecting engineer in London following a career with British colonial railways. Garratt first approached Kitson and Company with his design, but his idea was rejected, perhaps because that company were already committed to the Kitson-Meyer articulated design. He then approached Beyer, Peacock and Company, which were only marginally more interested.

The first Garratts

In 1907, Beyer, Peacock and Company submitted a proposal for a gauge Garratt to the New South Wales Government Railways, which was not proceeded with. The following year a design for a gauge Mallet locomotive was submitted in reply to an enquiry from the Government of Tasmania. The company then proposed a Garratt design based on, but a little heavier than, the design for New South Wales, with capacity to negotiate curves of radius and 1 in 25 gradients. The proposal was accepted, and two locomotives were built in 1909, which became the K class.

The Tasmanian Railways stipulated two features that were not in Herbert Garratt's original concept. The first was a compound configuration in which two high-pressure cylinders were on the rear engine unit and a pipe led to two larger, low-pressure cylinders on the front engine unit. The second was to have the cylinders facing inwards, which would reduce the distances between both the main steam pipe and the high-pressure cylinders and between the high-pressure and low-pressure cylinders. The latter feature made the locomotive unnecessarily complicated and placed the high-pressure cylinders directly underneath the cab, making it uncomfortably hot for the crew on the rare days when Tasmania's West Coast Range was warm. Only one more Garratt locomotive was produced with compound propulsion (by Beyer, Peacock & Co. in 1927 for Burma Railways). The company built no more Garratts with inward-facing cylinders, but two Garratts operated by the Southern Fuegian tourist railway at Ushuaia in Argentina, largely based on the K class, have that feature.

Early design and construction difficulties involved the steam-tight flexible connections between the boiler unit and the power units. However, Beyer, Peacock's engineers solved them after studying a description of the spherical steam joints used on a Fairlie locomotive.

Darjeeling Himalayan Railway

The third Garratt (another , like the first two) was built in 1910 for the Darjeeling Himalayan Railway and given the class letter "D". The power output was intended to be double that of the line's existing locomotives, but only a 65% increase in loading was achieved.

First main-line class

In 1911, Beyer, Peacock & Company built six Garratts for the Western Australian Government Railways. The M class were followed by the Ms class and the Msa class. These locomotives formed the pattern for the Victorian Railways narrow gauge G class and the Australian Portland Cement Garratts. The final built to a Beyer-Peacock design, in , were eight gauge South African Railways Class NG G16 locomotives.