Other equivalent classifications are:
UIC classification: 1DDD2 (also known as German classification and Italian classification)
French classification: 140+040+042
Turkish classification: 45+44+46
Swiss classification: 4/5+4/4+4/6
The equivalent UIC classification is to be refined to (1'D)D(D2') for these engines.
Only one 2-8-8-8-4 was ever built, a Mallet-type for the Virginian Railway in 1916. Built by Baldwin Locomotive Works, it became the only example of their class XA, so named due to the experimental nature of the locomotive. Like the same railroad's large articulated electrics and the Erie Railroad 2-8-8-8-2s, it was nicknamed "Triplex".
An overview of Triplex engineering is given at Triplex (locomotive).
The XA was unable to sustain a speed greater than five miles an hour, since the six cylinders could easily consume more steam than the boiler could produce. The tender had a four-wheel truck at the rear to help guide the locomotive into curves when drifting back downhill after pushing a train over the hill.
The XA was sent back to Baldwin in 1920 and was rebuilt as two locomotives, a 2-8-8-0 and a 2-8-2. Unlike their progenitor which lasted only a few years in service, these two locomotives remained in service until 1953.
Pennsy Multiplex 2-4-6-8-10-12
During WW2, the largest Steam Locomotive was unveiled. The Multiplex 2-4-6-8-10-12 and this locomotive was a quadruple steam locomotive with it having the locomotive connected to one tender and a separate tender behind. This locomotive was either cut up into smaller engines like a 2-8-8-0 or 2-8-8-2 or scrapped entirely.