Under the Whyte notation for the classification of steam locomotives, a 2-8-8-8-8-2 has two leading wheels, four sets of eight driving wheels, and two trailing wheels. Because of its length, such a locomotive must be an multiplex locomotive. It is longer than a normal articulated locomotive; the fourth set of drivers is located under the tender.
Other equivalent classifications are:
UIC classification: 1-D-D-D-D-1 (also known as German classification and Italian classification)
AAR classification: 1-D-D-D-D-1
French classification: 140+040+040+041
Turkish classification: 45+44+44+45
Swiss classification: 4/5+4/4+4/4+4/5
The equivalent UIC classification is to be refined to (1'D)DD(D1').
This type of articulated locomotive was never built, the only proposal for such a locomotive came from the United States. George R. Henderson was granted US Patent 1,100,563 for a quadruplex locomotive,[1] in June 1914. The patent was assigned to the Baldwin Locomotive Company. Baldwin presented the design to the Atchison, Topeka, & Santa Fe Railway (AT&SF), which was a strong proponent of compound locomotives in the 1910s.[2][3]
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).[4]
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).[5]
Due mostly to its extreme length the design included a number of mostly untried innovations:
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.