STMFC,
A couple notes about the excellent Rapido HO NP boxcars in the 100000-13999 series. 

The January issue of Railroad Model Craftsman has a nine page, 20 photo article by Chris Frissell that describes simple but highly effective weathering for this common steam era car. The article leans a little more towards the 1960s, but many of the techniques could apply to the Steam era.

Additionally, just a reminder, the NPRHA ordered custom road numbers from Rapido for the second run, and those acclaimed boxcars can be found in the NPHRA store for prices well below MSRP:
Modeling - Cars - NPRHA Company Store

Finally, the NPRHA also offers Cartograf printed HO decals including authentic chalk marks and common stencils for this car series, here:
Modeling - Decals - NPRHA Company Store

Thanks,
Dean ONeill
NPRHA Modeling Committee

PS - want access to over 5000 Northern Pacific Railway steam locomotive photos, along with over 100 elevation and section drawings? You need to See It All!:
Vista Dome Package 2020 - NPRHA Company Store