Like another anon said you wanna look for primarily 40' rolling stock. These can be single-door boxcars, single-dome tank cars, 33' hoppers, and gondolas/flat cars. Stuff like pic related, my small collection of 40s/50s rolling stock.
As for the railroad itself, the Northern Pacific covered the northwestern part of the US (Washington state, Minnesota, Idaho, North Dakota) and even a bit of Canada. Besides Northern Pacific stock you might wanna look into grabbing Great Northern, C&NW, Union Pacific, CB&Q, and the Milwaukee Road stock for stuff that would be near the Northern Pacific. Although you can get away with purchasing something like a NYC/Pennsylvania/Norfolk & Western/some other eastern-US railroad freight car here and there since these things went everywhere (Hell this weekend I saw a UP locomotive on my drive through New Jersey in what's usually Norfolk Southern/CSX trackage)