Triplex locomotive

In June 1914, George R. Henderson was granted US Patent 1,100,563 for a quadruplex 2-8-8-8-8-2 locomotive,[7] which was assigned to the Baldwin Locomotive Company. Baldwin submitted the design to the Atchison, Topeka, and Santa Fe Railway (AT&SF), which in the 1910s was a strong proponent of compound locomotives.[8][9][10]

This would have been, in 1913, by far the largest steam locomotive ever proposed. In quadruplex form, it would have been 129 feet 10+1⁄2 inches (39.586 m) in overall length, total weight of about 885,000 pounds (401 t), with tractive effort of 200,000 pounds-force (890 kN).[11]

Patent application drawing for the Baldwin quadruplex locomotive, 1913

The Quadruplex was to comprise three articulated engines of 8 driving wheels each beneath the locomotive itself, and a fourth engine beneath the tender. As a compound locomotive, engine cylinders 7 and 9 (as numbered on the above image) would receive high pressure steam to drive the first and third engines, each would exhaust as low-pressure steam to power cylinders 8 and 10 on the second and fourth engines. Both sets of low-pressure cylinders would then exhaust direct to atmosphere through stacks 33 and 38. The drivers had a diameter of 60 in (1524 mm).[12]

Due mostly to its extreme length the design included a number of mostly untried innovations:

• An engineer’s cab (24) at the very front, as well as a fireman’s cab (23) behind the firebox (17). Communication between the cabs was proposed as cable- or rod-operated signalling devices, similar to the engine order telegraph used on steamships, and possibly a voice pipe
• A jointed boiler with a flexible coupling (16) allowing the boiler casing to flex laterally on track curves. Such an accordion joint was already in use on a 2-6-6-2 locomotive of the AT&SF.[12]
• Two separate boilers, served by the single firebox: The front boiler (21) to supply the front two engines, the rear boiler (20) to supply the rear two engines. Working pressure of the boiler would be 215 psi (15 bar).[12]
• A turbine-driven extractor fan (26) within the smokebox (25) was intended to maintain a constant draft through the flues of both boilers. This was because Henderson had calculated that a conventional blast pipe utilizing steam exhausted from the low-pressure cylinders would have been inadequate to provide a sufficient draft until the locomotive was in motion.

By the time the patent was granted, the experience of the existing triplexes and jointed-boiler locomotives had shown the shortcomings of these designs, and the quadruplex did not proceed to construction.[8]

The only quadruplex locomotive built was No. 2096 for the National Railway Company of Belgium , built by the Ateliers Métallurgiques de Tubize in Belgium. It was also the first locomotive ever built with Franco-Crosti boilers. The locomotive consisted of three articulated sections with a total of four engines. The middle section, with two engines with four wheels each, carried the two steam boilers, which operated at 14 bar. They were arranged at a slight angle to allow the two fireboxes to be placed side by side in the centre.[13]

Longitudinal section and schematic drawing of the articulated locomotive system Franco-Crosti for the National Railway Company of Belgium, 1932

The two end units each carried a Franko-Crosti flue gas preheater. The connections between the boilers in the centre section and the preheaters in the end sections were expected to have problems with outside air being drawn in by leakage, so the spherical connections were designed with labyrinth seals.[14]

Belgian quadruplex steam locomotive No. 2096

The water tanks were also in the end sections, which were driven by six-wheel engines. All engines were single-expansion engines. The 20 drivers were supplemented by 10 idlers. The total weight of the three-part locomotive was given as 230 t (according to other sources 248 t), the adhesion mass as 164 t.[13]

In the middle section were two identically cabs, which were used depending on the direction of travel, as the 29 m (31 maccording to other sources) long locomotive could not use turntables due to its length. Each of the two boilers had its own coal box and was operated by its own fireman. The coal boxes were located to the side of the long boilers. While one fireman could communicate directly with the driver, the other fireman's line of sight to the driver was blocked by the coal box, so communication was by audible signals only.[13]

The locomotive was delivered in 1932, but was never actually used. The locomotive's tractive forces were so high that the chain couplers broke. The reason for building the locomotive is unclear, as it was far beyond the needs of the Belgian State Railways for freight transport, and could not be used for passenger transport due to its low top speed of 60 kph. The sheer size of the locomotives caused problems with stabling, the large number of coupling wheels, cylinders and seals on the movable connections between the preheaters and the main boilers were additional maintenance problems, apart from the fact that the locomotive required two firemen instead of one. The locomotive was exhibited at the 1935 Brussels World's Fair and was retired shortly afterwards. Some believe that the locomotive was built as a proof of concept for the project of a larger hexaplex locomotive with the same body for the Russian market.[15]

Beyer, Peacock and Company also applied for a patent for a quadruplex locomotive in 1927. Based on their successful Garratt locomotive design, it was called the Super-Garratt. The standard gauge locomotive was designed as an articulated 2-6-6-2 + 2-6-6-2 for the North American railroads and would have been built in conjunction with the American Locomotive Company (ALCo).[16] The starting tractive force of 200,000 pounds-force (890 kN) of the 460-ton locomotive would probably have been at the limit of the couplers in use at the time. Like a Garratt locomotive, it had a centre frame with boiler, firebox and cab, which sat on two subframes at each end. Each subframe, like a Mallett locomotive, contained a fixed set of driving wheels and, at the end of the locomotive, a pivoting bogie with another set of driving wheels. In addition to the standard gauge version, a 3 ft 6 in gauge version was planned for the South African Railways (SAR), but neither version was built.[17][18]

Rendering of a Super Garratt locomotive

Side view of a standard gauge Super Garratt locomotive

A quintuplex version (2-8-8-8-8-8-2) was also included in the George R. Henderson’s patent application for the quaruplex version. The design was based on the quadruplex, with the fourth and fifth engines under an extended, articulated tender.[9][10][19]

An even larger 2-10-10-10-10-10-2 variant appeared as an artist's impression in the August 1951 issue of Trains magazine. However, this idea seems to be speculative on the part of both the magazine writer and the artist, perhaps because AT&SF already had a fleet of 2-10-10-2’s in 1913. There is no evidence that either George Henderson or Baldwin suggested such a version.

There was a Belgian project for a hexaplex locomotive for the Russian market, based on the Franco-Crosti quadruplex locomotive built for the Belgian State Railways. It had the wheel arrangement 2-4-4-2 + 2-8-8-2 + 2-4-4-2 and made full use of the Russian loading gauge and broad gauge. The total weight of the 43-metre-long locomotive was to be 410 tonnes, of which about 320 tonnes would have been available for adhesion. The water tank capacity was to be 60 m3, the coal capacity 15 tonnes.[14]

Longitudinal section and schematic drawing of the hexaplex locomotive system Franco-Crosti for the Russian market, 1930s