As this railroad was mainly built to haul heavy tourist trains into the Catskill Mountains, it needed powerful passenger locomotives. In 1906 and 1907, ALCO-Schenectady built two batches of six Ten-wheelers each which were numbered 30 to 41 by their operator. Their drivers measured 63 inches and the cylinders were 20 inches in diameter and had a stroke of 26 inches. They had slide valves which were actuated by Stephenson valve gear.

Despite a starting tractive effort of 28,000 pounds, they sometimes still needed helpers up the steep mountains. In addition to the passenger trains, they also had to haul produce like milk what still required the ability to reach adequate speeds. Only four were superheated in 1923 and 1924. In the same process they received a cylinder diameter of 21 inches and piston valves, but retained their Stephenson valve gear. When the U&D was integrated into the New York Central in 1932, they became class FX No. 808 to 818. The last ones were retired in the late forties.