I'd be interested to know waht class of NYC pacifics were built for Freight service. I know the Canadians had a lot of these but they were built with smaller drivers for dual and/or freight service.
Main Line passenger locos like the PRR K4 (and nearly all K Class) had 80 inch drivers. This gave them the speed capability to run 100 MPH or more if needed, but it also reduced the tonnage they could haul. In Freight service a passenger loco would tend to be slippery and not capable of drag freight work.
On the other side, passenger locos were generally equipped with different brake systems (in many cases), signal lines to communicate with the conductor in the rear, and steam lines going back to heat the passenger cars. Freight engines had none of these features and so were generally not used in passenger service unless to rescue or fill in.
I would guess as steam wound down, there were instances of a passenger loco doing something on a small freight. But I think as a rule, a pacific, especially with 80 inch drivers, was reserved for passenger service.
