The interesting thing about 60mph is that it would have been a ‘passenger’ limit; PRR had the 50mph speed limit for freight firmly established in this period.
Could you be misreading 80mph as 60mph? The New York Division forward limit for M1/Mia in 1946 was 40mph light engine, 75 with train, but by 1952 the ‘with train’ rating had been raised to 80mph. Strangely, on the Middle Division in 1952, the light speed was 50mph, but the ‘with train’ speed was only 70mph.
Perhaps listing the exact ETTs and pages in which the slower ratings appear might be helpful, since there are so many of them to go through… not that I’m complaining!