Dave: If you are interested in mainline railroads in the Northwestern US, the Northern Pacific was the first major railroad in this area, followed by the Union Pacific in Oregon and Washington, the Great Northern along the northern most line through North Dakota, Montana, Idaho and Washington. The last major transcontinental line to reach the northwest was the Milwaukee Road, early in the 20th century.
The Milwaukee went out of business in the early eighties. The Northern Pacific, Great Northern and Burlington Route merged into the Burlington Northern (BN) in the mid 70's which subsequently merged with the SantaFe to become BNSF. Union Pacific still exists in the Northwest.
There were other shortlines and logging railroads by the hundreds over the years.
For NP models, try
companystore@NPRHA.org There is also a Great Northern Historical Society and one for the Milwaukee Road ( Officially the Chicago, Milwaukee, Saint Paul and Pacific Railroad) Also try looking up The Spokane, Portland and Seattle Railroad, it was a joint venture between the NP and GN
This is a quick and generalized overview of some of the major carriers in the Northwest. Others can fill in details for you. If there is something specific, let us know. Best Regards; Jim Krause