Herbert William Garratt thought about how a powerful and flexible steam locomotive for narrow gauge railways should look like. His design consisted of a locomotive divided into three parts. Two running gears each contained a complete steam engine, on one side carrying a water tank and on the other a combined water and fuel tank. The third part consisted of a bridge frame that carried a boiler and a cab. The bridge frame was suspended centrally between the two running gears and supplied the two engines with steam.
For the implementation, he first turned to Kitson & Co., where he was unsuccessful. This can probably be explained by the fact that they were already producing articulated locomotives of the Kitson-Meyer design. He had more success with Beyer, Peacock & Co., so that the design is now also known as the Beyer-Garratt.
The first locomotive that can be called a Garratt was built for the North East Dundas Tramway in Tasmania and still had some deviations from the later standard form. It had the wheel arrangement 0-4-0+0-4-0 T and is known today as Tasmanian Government Railways class K. At the special request of the customer, the two locomotives worked in compound action and had the cylinders on the inside. This arrangement was very complex and also caused problems with the cylinders directly under the driver's cab. Later Garratts had single steam expansion and outside cylinders. The first model built for mainline service was the West Australian Government Railways class M with a 2-6-0+0-6-2T wheel arrangement.
The biggest advantage of the Garratt was that the designer had a free hand in designing the boiler and there were no frames, axles or wheels in the way. This was particularly important on the narrow gauge, because they could create a wide firebox. In addition, the firebox could be manufactured inexpensively with straight side walls and the ash pan was more accessible. The boiler barrel could also be made short and with a large diameter in order to be able to develop optimal performance.
One disadvantage the Garratt shared with tank locomotives was the reduced adhesive weight as supplies dwindled. The train loads either had to be planned in such a way that the locomotive could pull them even when the supplies were almost empty, or water had to be taken more often than actually necessary to keep the weight high. In addition, during construction, care had to be taken to ensure that the space between the smoke box and the front water tank is large enough to open the smoke box door. This could be ensured by an indentation in the water tank or a sufficiently large distance between both.
The running characteristics were characterized by the fact that the chassis was symmetrical and high speeds could be run in both directions. Especially on lines with many tunnels, they often ran with the cab first, i.e. “bunker leading”. Because the boiler, unlike that of the Mallet, didn't swing outward in turns, tighter curves could be negotiated and the running was better at higher speeds. Furthermore, the Mallet tended to get unstable when coasting downhill at high speeds, a problem that didn't exist on the Garratt.
A special design was the Union Garratt. The front water tank was still on its own frame, but the rear water and coal tanks were on the frame that also carried the boiler and driver's cab. The rear chassis was designed as a bogie. So it was a hybrid of Garratt and Modified Fairlie.
Most Garratts were built for Africa and many for Australia and South America. Others were used in Asia and only a few in Europe. This can be attributed to the fact that there were many narrow-gauge lines in the main regions where they were used, which had many curves and were often only designed for low axle loads, which made the Garratt an ideal option. Since there were large quantities of raw materials to be transported in these countries, large Garratts were built, especially for the Cape gauge and meter gauge, which were more powerful than the most powerful steam locomotives in Europe. The most powerful Garratt was the class 59 of the East African Railways with the wheel arrangement 4-8-2+2-8-4T. Although it only ran on the meter gauge, it had a service weight of over 250 tons, had a grate area of 72 square feet and had a starting tractive effort of around 83,000 pounds.
Since the general speeds were often low, the Garratts were mostly used for mixed traffic. Depending on the rail network, vehicles with larger drivers were also built, which were then primarily used for passenger trains. So it was possible that a railroad would purchase a small-wheel 4-8-2+2-8-4T “Double Mountain” for freight trains and a large-wheel 4-6-2+2-6-4T “Double Pacific” for passenger trains. The standard-gauge 231-132.AT and BT from PLM Algérie with 71 inch driving wheels are considered the fastest Garratts at 82 mph.
After the patent for the design expired, other manufacturers in other countries also built Garratts. For example, the South African Railways had a larger number manufactured by the German companies Hanomag, Henschel and Maffei. The last Garratt built was the South African Railways Class NG G16, the last of which left the Hunslet-Taylor factory in South Africa in 1968 and were fitted with boilers from the parent company in England. Like the very first Garratt, these ran on the small two-foot gauge.
A total of 1,704 Garratts were built, of which 1,124 went to Africa alone. Especially there, some operators were still dependent on their services for a very long time. In Zimbabwe, for example, Garratts were still used regularly in front of passenger trains after the turn of the millennium. It is currently assumed that there are still around 250 existing locomotives in a wide variety of conditions. Of these, however, there are probably fewer than 100 in reasonably decent condition and around 15 are still roadworthy.