The process of historical research

One of the reasons I wanted to model early days of the BN was the amount of research involved to accurately model the prototype. Some times its more frustrating knowing that you want to model a specific era, and location, but you don't have the historical information. Although research is an ongoing process that gets richer and richer the deeper you dig it can be frustrating at times. Modeling a specific prototype and era has plus and minus. The plus is you get to build everything yourself. The minus is you get to build everything yourself! That requires a lot of information, which isn't always easy to find.
So here's a list of some of the useful places that I found historical information for just the towns and buildings.
I'm sure it will grow.
Northern Pacific Archives, St. Paul, MN, held by the Minnesota Historical Society
Northern Pacific Railway Historical Association
GN-NP Joint Archive
Sanborn Fire Insurance Maps specific to your state, county, or city. These can be hit and miss depending on the era your modeling. Most local libraries have access to the maps, most of which are online. The library of Congress has a lot as well that are not digitized.
Washington State Archives Depending on what the archives have for specific properties you may find structure drawings, pictures, plat maps, etc. With this you need to know the parcel number of the property. This can be obtained through the local county websites
Local Historical Societies. I've found if you call ahead and visit their collections in person you'll get some good information. Again this can be hit and miss.
Washington State Historical Preservation Office
Various specific online forums; for my case I try and follow anything BN related, NP, and GN related.

Wa. State Tax assessment files

Wa. State Tax assessment files

Each one of these archives has a slightly different method of acquiring information. Knowing how to search is just as important as knowing where to look.