From TrainSpottingWorld, for Rail fans everywhere
The Pennsylvania Railroad's lone S1 was the only 6-4-4-6 ever constructed.

A 6-4-4-6 steam locomotive, in the Whyte notation for describing locomotive wheel arrangements, is one with six leading wheels in a leading truck, two sets of four driving wheels, and six trailing wheels in a trailing truck.

The equivalent UIC classification is 3'BB3'.

The largest rigid frame passenger locomotive ever built, only one locomotive was produced to this arrangement, the Pennsylvania Railroad's sole class S1 of 1939. It was a duplex locomotive and is often referred to as the Pennsylvania Type. This experimental locomotive was exhibited at the 1939 New York World's Fair, and was afterward placed in limited service between Chicago, Illinois and Crestline, Ohio. The locomotive was too large to work elsewhere in the system. Pennsylvania Railroad executives hoped that the locomotive could haul 1,000 tons at 100 miles per hour, but this goal was not reached. It was capable of very high speeds however, although no documentary evidence has so far surfaced to add credence to stories of record-breaking performance.

cs:Lokomotiva S 1-6.100