I would add E-Bay (for photos, papers and maps) of localities, industries and railroads.  And just doing a cold image search for localities, industries and railroads.  Yes, it takes time (minutes) but if you cast a big enough net, sooner or later you will catch a fish.

Don't forget City Directories to find the names of industries and businesses and some libraries have them available on-line.  If you know the address you can find the name of the business or if you know the name of the business you can find the address.  Plus the directories have advertisements so you can sometimes find company logos that you can use for signage on the layout.  Just another research tool that is available. 

Robert

Neither Sanborn nor USPS maps are accurate for RR track placement. Sanborn didn't insure the RRs so they didn't bother with RR track accuracy. The USPS topographic maps being done at 1"=2000' 1 line can equal 2-3 tracks making accuracy near impossible..

Andy Jackson

Santa Fe Springs CA

I think we all pretty much agree that only the actual railroad engineering drawings are 100% correct, but even then they are only accurate during their snapshot in time.

Historical USGS 1:24,000 maps are close enough to get you into the right area and for those who are good enough map readers, abandoned roadbeds can be detected which can lead a person on sometimes what seems like a never ending hunt for more information. I'm satisfied with the USGS map detail unless it's in a large yard area. Single passing sidings are normally accurately mapped.

I've found the Sanborn maps for the areas in which I am interested to be 100% correct with regards to industrial spurs and sidings. I've both field checked them and compared them to historical and current aerial photographs. I've even found notations on some of the maps concerning certain aspects of some of the trackage, such as trestle details.

As several of the previous posts have said, it's best to use all of these resources because, at least in the upper mid-west where I'm concentrating, it's rare to find a spot where all of them are available.