When I started visiting the ng. 1960, I do not remember seeing any late night trains. Some trains got in close to midnight, but the calls at least in my limited experience were in the early to late morning. It was down to a pretty regular two "cycles" per week operation.
However, from an account of operations during the mid-50's that Earl Knoob compiled, it was a 24 hour operation straining to handle the business with the resources available. Trains were called when power and cars were ready, regardless of the time of day or night.
My guess is what John is showing us is closer to that peak situation than to what was happening just three years later.