Penn Central used just about any 2000-2500 h.p. road switcher on the Catskill Mountain Branch - GP-20's, Alco's and an occasional GE. "The Old Up and Down" by John Ham and Robert Buccenec documents the last years very well. The photo of an F-7A is unusual. The only cab units I had seen on the branch, up to 1965, were two Alco FA-1s, including the 1032, that took the Selkirk wreckers to South Gilboa in February 1958 to rerail a snow plow that derailed behind the Blue Silo Farm.
When the section from Bloomville to Oneonta was abandoned in 1965, KO-1 and OK-2 became KB-1 and BK-2 (Oneonta in the symbols became Bloomville). The train almost always had Stamford and Hobart cars, and they went to Bloomville only if there was a good reason. If they could make the trip within the 16 hour law, they would go to Bloomville and then return to Stamford to tie up and take their rest at the Delaware Inn. Otherwise they would go to Bloomville after tie up, and try to make it back to Kingston under the law.
When they ran to Oneonta, KO-1 usually had two units, RS-2's and -3's, mostly to save time by not having to double Pine Hill, and to avoid turning a single unit on the Oneonta wye. After the cut back to Bloomville, they generally had only one unit, because the business had fallen off, they didn't have as far to run, and there was no place to turn an engine west of Kingston. They just went home in reverse.
I don't think the line was ever really cut back to Stamford. The business at Bloomville just dried up, and they didn't run down there anymore. The Penn Central Estate still owned the railroad to Bloomville, and Conrail operated it as a subsidized light density line as far as there was any business, for six months following the Conrail conveyance on April 1, 1976. The Catskill Rail Committee bought everything from the Ulster County Line to Bloomville from the Penn Central Estate.
By the way, are you related to a former Supervisor of the Town of Kortright?