The worlds first and only locomotive with a 6-8-6 wheel configuration was introduced in 1944. The PRR S2 direct-drive steam turbine powered locomotive was a unique experiment in motive power. Its six-wheel leading truck, eight drive wheels and six-wheel trailing truck made for a massive running platform that was quite efficient when the S2 was at speed. Originally, the S2 was to be a 4-8-4, but wartime restrictions on the usage of lighter steel alloys forced the use of heavier alloys - eventually driving the weight of the entire locomotive to need larger leading and trailing trucks.

The S2 is outfitted with two steam turbines. One for each direction of travel. The large boiler sports a Belpaire firebox and the turbines exhaust was channeled through four pipes in the smoke box. A Worthington-style feedwater heater honed the S2s efficiency further.