Hudson was the name given to the 4-6-4 steam locomotive wheel arrangement. The New York Central was the first to use locomotives of this type in North America.
Although the Milwaukee Road was the first to design such locomotives, naming them Baltics, they didn't get built until after the NYC's Hudsons. The name Baltic for this type was also used in Europe.
Alvin F. Staufer interviewed the Hudson's designer and the NYC's superintendent of motive power, Paul Kiefer, in 1961, and asked about the naming of the type. Kiefer's answer is recounted in Staufer's book, Thoroughbreds:
I asked Pat [Patrick E. Crowley, NYC President, All Lines] if we should name the engine or if he cared about that at all. We were already calling the L class 4-8-2s Mohawks, after the Mohawk Valley and Indians. And then, I'll never forget that moment, he just looked at me; the sun was shining in from the West, it was late in the day. He swung aroud in his huge brown leather chair, away from me. He stared out the window for the longest time. He swung back and stared at me, his chin in his hand. Finally he spoke, "Let's call her the Hudson, after the Hudson River." I agreed immediately (not that it mattered) and that's how it was. The name stuck. It was a natural.
The Hudson came into being because the existing Pacific power was not able to keep up with the demands of longer, heavier trains and faster speeds. Given the New York Central's axle load limits, the Pacific type could not be made any larger; a new locomotive type would be required to carry the larger boiler that Kiefer's research indicated was needed.
Lima's conception of superpower steam as realised in the 2-8-4 Berkshire type was the predecessor to the Hudson. The 2-8-4's 4-wheel trailing truck permitted a huge firebox to be located after the boiler. The resulting greater steaming rate ensured that such a locomotive would never run out of power at speed -- a common failing of older locomotives. Applying the ideas of the freight-minded Berkshire type to the Pacific resulted in a 4-6-4 locomotive.
The New York Central produced one prototype, #5200, and subjected her to intensive testing. Passing with flying colors, a fleet of 205 Hudsons, classified J1, were built. Future developments included the J2 for the Boston & Albany Railroad, a NYC subsidary, and the J3 'Super Hudson' in 1937, with every modern appliance and innovation the railroad could put on it.
They were some of the best known steam locomotives ever. A number were streamlined and featured prominently on New York Central advertising for trains like the Empire State Express and Twentieth Century Limited.
Forte of all Hudsons was power at speed. They were poor performers at low speed and the presence of a booster on the trailing truck was an absolute necessity for starting. For this reason, they were generally favored by railroads with flat terrain and racetrack-like routes. After the NYC, the Milwaukee was the biggest fan of the Hudson type; the Santa Fe also had them. Few roads with hilly terrain acquired any.
A booster, though, was a complicated piece of machinery, liable to trouble. Slowly, the booster fell out of favor with railroads, including the NYC. Instead of a complicated booster, isn't it just easier to have an extra pair of driving wheels, and thus better traction? Also, in its turn, the Hudson was beginning to be overtaken by the weight of trains, just as the Pacific before it.
Trials of dual-purpose 4-8-2 Mohawks in passenger service sealed the Hudson's fate. A larger locomotive was clearly now required. The Mohawk was nice, but it was still more suited to lower-speed hauling than high-speed power. In 1944, the New York Central recieved permission from the War Production Board to build a new, high-speed locomotive of 4-8-4 type, combining all the advantages of the Hudson with those of the Mohawk. Many other railroads had taken to the 4-8-4 in the 1930s, generally calling them Northerns after the Northern Pacific who had first adopted them. By being a late adopter, the NYC had the chance to build on everyone else's experience.
That locomotive proved to be exceptional, and the type on the NYC was named the Niagara. They were arguably the epitome of steam passenger power in the United States, and possibly the world. Since only 27 were built, however, they only took over the top flight of trains; most Hudsons labored until the end of steam on the New York Central.