The 4-6-4 Hudson was a natural progression from the 4-6-2 Pacific, the standard passenger engine of the early 20th century. By the Roaring 20s, the popularity of rail travel and the conversion of passenger fleets from wood cars to steel was taxing the ability of Pacifics on many railroads. The larger, 4-wheel trailing truck on a Hudson allowed a bigger firebox that generated more steam, and thus, more power. Some roads went even further and added another pair of drivers to create a 4-8-4 Northern, but the Hudson was big enough for roads in primarily flat territory - most notably the New York Central with its famous Water Level Route from New York to the Midwest. As the most prominent user of 4-6-4s, the NYC also got naming rights, and the wheel arrangement was named after the Hudson River flowed alongside its tracks.
When the nation slid into the Depression in the 1930s, passenger travel fell off dramatically. In an effort to revive their fortunes, railroads turned to a new look -streamlining - to make train travel look modern and exciting. In some cases that meant entirely new streamlined trains like the Union Pacific M-10000 shown elsewhere in this catalog. Other railroads turned their efforts toward streamlined steam. At its worst, streamlining produced steam locomotives that looked like upside-down bathtubs. But at its best it yielded results like the New York Central's Dreyfus Hudsons and the Norfolk & Western \"J\".
For less well-heeled railroads, however, streamlining often meant some added sheet metal on existing steamers to gussy them up. So it was with the Wabash. Short of passenger power in the mid-1940s, it rebuilt seven older 2-8-2 freight engines into semi-streamlined Class P1 Hudsons. The last engines built, nos. 705 and 706, sported tall \"elephant ear\" smoke deflectors on either side of the smoke box, designed to deflect smoke up and out of the engineer's line of vision. Decked out with the Wabash \"Follow the Flag\" logo, the P1s worked in passenger service until retirement in 1956.