I just did a search for railroad lines in the northwest. The line from Canada going south through western Washington into western Idaho matches the location of a railroad line. My guess is the bad guys will or have employed a container from a cargo ship that is moved to a railroad car to bring a bomb into the U.S. The available railroad paths take such cargo right past my home in the Pocatello area.
I tried to find a suitable link to post that shows the maps with the railroad lines, but most of this is offered with online "viewers" that aren't suitable to include inline here.
A few days ago, there was a report of a missing shipping container in Canada, all I recall is the 3 days that it took for them to make the report.
I don't recall who posted it. I am sure it was on this thread.