A multiplex locomotive is a steam locomotive that divides the driving force on its wheels by using multiple pairs of cylinders to drive multiple driving wheel set groups. Such a locomotive will necessarily articulated if it has more than two sets of driving wheels. There were locomotive projects with three, four, five or six sets of drive wheels. However, these locomotives were never built, except for four triplex locomotives in the United States and one quadruplex locomotive in Belgium.
The names of the subgroups of multiplex locomotives is derived from the number of drive wheel sets and is composed of the numeral prefix and the word locomotive. However, locomotive is often omitted if it can be inferred from the context.
Duplex locomotives
The terms duplex locomotive and Divided drive locomotive are used to describe locomotives with two drive wheel sets mounted in a rigid frame.
The concept of the duplex locomotive originated in the early development of the compound locomotive, in which the high-pressure cylinders worked on drive wheels or drive wheel sets that were not coupled to those on which the low-pressure cyllinders worked. Later, the concept was used to limit the force in the connecting rods and the hammer blow, i.e. the dynamic forces of the steam engine drive acting on the rail. This concept was used most notably by the Baldwin Locomotive Works in North America in the early 1930s. The most successful duplex locomotive of this type was the Pennsylvania Railroad's T1 class.
Articulated locomotives with two drive wheel sets were typically of the Mallet, the Garratt or the Meyer type. These types are not normally considered to be duplex locomotives.
Triplex locomotives
Triplex locomotives are locomotives with three drive wheel sets. The Baldwin Locomotive Works built three 2-8-8-8-2 triplex locomotives for the Erie Railroad and one 2-8-8-8-4 for the Virginian Railway. All the triplex locomotives built were of the Mallet type, but with an extra set of driving wheels under the tender. They could only be used as slow pushers because of the enormous tractive effort was too much for the car couplings to withstand and the boiler did not produce enough steam to operate the locomotives at higher speeds.
The triplex locomotives for the Erie Railroad were built between 1914 and 1915 and were given the classification of P-1. They were not considered highly successful and scrapped till 1933.
The triplex locomotive built for the Virginian Railway, as No. 700, in 1916 was given classification of XA. It was considered unsuccessful as well and was sent back to Baldwin Locomotive Works where it was taken apart in 1920 and converted into a 2-8-8-0 and a 2-8-2.
Quadruplex locomotives
Quadruplex locomotives are locomotives with four drive wheel sets. There was a project by Baldwin Locomotive Works and one by Beyer, Peacock and Company for the United States, both of which were never realised. The only quadruplex locomotive built was the Belgian quadruplex SNCB 2096, which was not successful.
United States
In June 1914, George R. Henderson was granted US Patent 1,100,563 for a quadruplex 2-8-8-8-8-2 locomotive, 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.
This would have been, in 1913, by far the largest steam locomotive ever proposed. In quadruplex form, it would have been in overall length, total weight of about , with tractive effort of .
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).