I model the Algoma Central, a fallen flag regional in northern Ontario (Canada).
A quick look at what is or has been available in HO scale for this road, in no particular order:
FP9A locomotives - Rapido Trains (full road/unit specific detail)
FP9A locomotives - Intermountain (generic detail)
Steam generator car (CN version) - Rapido Trains
Baggage Car (CN version) - Rapido Trains
Coach (CN version) - Rapido Trains
SW8 - Life-Like Proto2000
GP7 - Life-Like Proto2000
52' Gondola - Life-Like Proto2000/Canadian Hobbycraft (stand in)
52' Gondola - Rapido Trains (Canadian version)
GP38-2 locomotive - Life-LifeProto2000/Canadian Hobbycraft (generic detail)
52' bulkhead flatcar (multiple runs, 2 paint schemes) - Walthers
"Ortner" rapid discharge hopper - Walthers (minor detail differences as the real ones were clones built by NSC)
90-ton triple hopper - TrueLine Trains/Canadian Hobbycraft (stand-in - special runs of custom decorated Atlas hoppers)
GP38-2 locomotive - Atlas (generic detail)
SD40 locomotive - Kato (generic detail)
SD40-2 locomotive - Bowser Trains (full road-specific detail)
SD40 locomotive (announced/upcoming) - Bowser Trains (full road-specific detail)
Steel caboose (CN version) - True Line Trains
Steel caboose (CP version) - Rapido Trains
Steel caboose - Atlas Trainman (totally inaccurate and stand-in, but offered in this road's colours)
40' gondola - Atlas Trainman (totally inaccurate and stand-in, but offered in this road's colours)
Steel caboose (CN version) - Overland (brass)
http://vanderheide.ca/blog/2016/05/12/an-rtr-algoma-central-more-or-less/
Canadian modelers are in a bit of a golden age with companies like Rapido and Bowser. True Line Trains was also a player but seems to have faded away.
This is all fairly modern stuff - to model the steam era would require a lot more work. As noted there's a varying amount of detail work to do in some cases. And there's still all sorts of weird stuff like the older wood cabooses, cylindrical body open hoppers, bulkhead gondolas and 40' pulpwood flatcars that are totally unique to this railway.
NP related - have you seen the new wood boxcars from Rapido which are an NP prototype?