On the Southern Pacific the later order for EMD F7 diesel units was designated FP7A with road #6446-6461 and presumably ran as passenger units - I cannot find any FP7B’s identified. However what was the livery of the F7PAs - was it Daylight? All the other freight F7A and F7B units I have seen in photos seem to run in Black Widow.
SP FP-7’s were delivered in “Black Widow” paint. The June 1958 paint standard shift to the “bloody nose” gray and scarlet scheme was applied to these units fairly quickly (within a year or so?) as they were passenger units. FP-7’s were A-units only. The reason for their existence was to provide additional space for a large water tank (train heat). B-units had enough space, so they werre never stretched the four feet that created an FP-7A. Indeed, SP’s order for FP-7A’s came with matching F-7B units. SP owned quite a few boiler-equipped B units. SP also owned and used in passenger service, F-7A’s with boiler controls (control of a trailing unit’s boiler) and passenger gearing. Several of these remained in passenger service until Amtrak.
Amtrak bought these engines from SP and used them for a few years,until Amtrak could purchase its own diesels.Amtrak leased some of these units back to SP where they ran in Amtrak paint.The FP7 was 4 feet longer than a standard F7 to make room for the steam heating boiler.The FP7 is my favorite diesel .[:D]