Northern Pacific Railway Equipment Rosters
If you are interested in NP Freight Cars the NPRHA has in it's collection the NP Equipment Ledgers.
These were used to keep track of equipment numbers, this set starts in the 1920s and goes to the 1980s (past the BN merger).
They include ALL rolling stock, Freight, Passenger, Locomotives, Cabooses, Work Equipment, etc.
They have been scanned and are on the NPRHA website: