The New York Central replaced branch line steam with diesels on a geographic basis. The Catskill Mountain Branch was dieselized in one shot in 1948. NYC bought their first Alco RS-1's, 8100's, for the CMB and the Wallkill Valley Branch so they could eliminate steam servicing and maintenance at Kingston and Oneonta.
Two U&D locomotive classes were taken by NYC in 1932 - the 20-class (actually Engines 19 through 29, except Engine 20, the inspection engine) and the 30-class, Engines 30 through 42. NYC put them all into Class Fx without distinction between the 20's and 30's, or between superheated and saturated engines.
Slightly off topic, but since Otto mentioned the Put -
I watched the dieselization of the Putnam Division in 1951. It all happened in about two or three months in the late summer and fall. We kids knew, of course, that the steam power would never be scrapped. It was too valuable, and they would certainly keep them in storage in case we ever had another war. Right!
I remember one cold, still winter morning when I was outside the old Ardsley School and I saw the smoke of an eastward train on the Put. It blew one perfect smoke ring. I never saw another one.
Gordon Davids