In view of the plans for a new electrified line, the Great Eastern tried to counter with a particularly powerful passenger tank locomotive. In order to be able to keep up with future electric railcars, the greatest attention was paid to comparable acceleration in order to achieve a reasonable travel time on the network with many closely spaced stations. Thus it was defined that a passenger train with a weight of 315 tons must be brought to a speed of 30 mph within 30 seconds. To meet these requirements, James Holden designed a locomotive of which only a single example was built.

The result was the first ten-coupled steam locomotive in Great Britain, which also had a very small driving wheel diameter for a passenger locomotive. If two cylinders had been used for the required tractive effort, the loading gauge would have been exceeded, so the locomotive was designed with three slightly smaller cylinders. While the outside cylinders acted on the third axle, the centre cylinder drove the second axle. Since the inside cylinder was also level with the axles, the connecting rod was wrapped around the first axle in a special, fork-shaped arrangement patented by Holden.

The boiler was designed as large as the loading gauge allowed. For this reason, the chimney, the steam dome and the safety valves had to be as low as possible. The Wootten firebox based on the American model allowed a large grate area and had to be divided into three areas because the wheels of the last two axles protruded directly into it. To achieve the necessary running characteristics, the first and last axles were installed with half an inch play on both sides and the middle one had no wheel flanges.

Although the axle load of the “Decapod” was within the permitted values, the five axles lying closely behind each other represented too great a load for the bridges. Since upgrading all the bridges on the lines used was not an option, there was no series production of the A55. In 1906 the locomotive was rebuilt to an eight-coupled tender locomotive known as the class A55R.

It got a significantly lighter boiler with a conventional Belpaire firebox and the centre cylinder was removed. Since the length of the locomotive was still large, the axles were spaced farther apart. In order to still be able to run through all curves without any problems, the first and fourth axles were fitted with radially adjustable bearings. This locomotive was now used to haul coal trains, although it did not display significantly greater efficiency than the three-coupled G58 class locomotives. So it was scrapped in December 1913.