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. The first Garratts had the wheel arrangement 0-4-0+0-4-0 and were built for the North-East Dundas Tramway, a 2' narrow gauge line of the Tasmanian Railways. They formed the K class and were numbered K1 and K2. One is still preserved on the Welsh Highland Railway.

Development

Beyer-Garratt

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.

Advantages

The principle benefit of the Garratt design is that the boiler and firebox unit are slung between the two engine units. This frees the boiler and firebox from the size constraints imposed where they are placed over the frames and running gear, as in conventional designs and other articulateds such as Mallets. Garretts can have a boiler with a greater diameter, which increases heating area and aids the production of steam. The boiler can also be shorter than other designs. In some loco designs, the boiler is so long almost no heating of the water occurs at the smokebox end of the boiler. A larger firebox promotes more efficient combustion of fuel and also increases the heat available to the boiler.

Garratts are known for their smooth running capacity. This compares with the Mallet, where the forward articulated unit tends to throw out as the loco rounds curves. As a result Garratts were capable of higher speeds, and were less likely to damage track. While most Garratts were designed for freight or mixed traffic, there were a number of passenger Garratt classes. In fact a Garratt holds the world speed record for an articulated locomotive.

Operationally Garratts have several advantages when used on light and narrow gauge railways. They are tank locomotives, thus eliminating the need for expensive turntables or wyes. They don’t need to be run through to terminals increasing operational flexibility. Because the engine units are separated by the boiler unit, the weight of the locomotive is spit over the two units. Therefore they can run over bridges that might not be able to support conventional or Mallet locomotives of similar weight.

Disadvantages

The major disadvantage of a Garratt (shared with all tank engines) 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). The weight of the water in the tanks can of course be predicted in advance, and with care this problem is not necessarily an insurmountable one.

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 when operating 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. Two crew died in Western Australia in the 1940's when an Australian Standard Garratt stalled in the state's only tunnel, the Swan View Tunnel.

Operational Issues

Garratt locomotives sometimes exceeded the operational capacities of the railways they operated on.

In New Zealand the Beyer-Garratt built 4-6-2+2-6-4 NZR G class locomotives were much too powerful for the drawgear then used in New Zealand, and short crossing sidings made it impractical to haul loads proportionate to the power of the locomotive.

The smooth operation at speed of the early Tasmanian 4-4-2+2-4-4 locomotives resulted in drivers exceeding speed limits, and subsequent unacceptable levels of derailment. This led to these locomotives early withdrawal.

Sources

• Garratt Locomotives of the World, A.E. Durrant
• A Complete List of All Garratt Locomotives, Gavin Hamilton
• K1 The World's First Garratt, Ben Fisher
• The Garratt Locomotive, Technology in Australia, 1788-1988
• Karamoja,a Kenya-Uganda Garratt Locomotive, garrattmaker
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See also

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Articulated locomotives | Garratt locomotives

Garratt