STMFC,
The NPRHA has developed a decal set in HO scale for the NP 10000 series boxcars, recently manufactured by Rapido.
The sheet includes unique stencils, common placards/forms, reweigh/lube stencils, paint patches, and also material to decal one car from scratch. The decals were printed by Cartograph in Italy, so quality/resolution is superb. We relied on steam-era photos to recreate 70% of the chalk marks. The included attachments give some examples, some of which MIGHT be applicable to non-NP modelers.
I would also like to add that the NPRHA store still has these RTR boxcars available, although they are going fast. The un-numbered cars can be decaled with this decal sheet. 
Decals (8$ shipped)
http://store.nprha.org/complete-decal-set-for-rapido-np-10000-series-boxcar
Models: ($37.50)
http://store.nprha.org/modeling/cars/

Thank you,
Dean O'Neill
Northern Pacific Railway Historical Association


PS - BIG THANKS to Rapido for the fantastic production job on this model!
PPS - The 36 and 48 inch NORTHERN PACIFIC Monads might be the best ever printed, so those could be used elsewhere.