When Did the PRR Retire Its 0-4-0 and 0-6-0 Steam Locomotives? - Trains Magazine

Those were among the least satisfactory services for steam power in the first place; they would have been kept 'in steam' only while a diesel alternative still cost more overall.  With line maintenance of the locomotives abolished, and formal fuel and water and ash-handling provisions removed, there would be increasingly little benefit, either in re-using costed-down assets or in operating savings, from using any steam power.

As noted, a great deal of steam power (I don't know what it was, but it's been carefully documented) was retained until at least 1959 against traffic increase or other prospective need.  Note that not one engine of it was ever resuscitated.

The next step, undertaken within three years, was to weed out all the oddball first-generation diesel and electric power that was nonstandard.  Reading between the lines, nearly anything 'steam' that might have survived that long would have been eliminated then.