Most if not all locomotive designs with four-wheel trailing trucks were the result of the desire for more power and the need for a bigger firebox. The 2-6-6-4 design was no different. In 1934 Baldwin constructed three of this wheel arrangement for the P&WV (Pittsburgh & West Virginia) who desired to increase the speed of their freight trains.
This wheel arrangement was made most famous by the Norfolk & Western who perfected this design. The N&W had the most powerful 2-6-6-4s and used roller bearings on the axles. The last five were also equipped with roller bearings on the side and main rods!
High-speed articulateds generally were of two wheel arrangements -- the 2-6-6-4 and the 4-6-6-4. Baldwin was the primary builder of the 2-6-6-4 while ALCO was the primary builder of the 4-6-6-4. In the end, the 4-6-6-4 wheel arrangement became more popular.