Has a trolleybus route ever been tried that far before? One of the reasons we had the Breda Dual Mode buses for the tunnel was because of the cost it would have been to have strung the trolleywire all the way to places like Auburn and Federal Way(and I doubt the wire would be great on an Interstate).
I am not sure if the STA has any routes that get close to the Idaho line. The closest point I remember they got when I was going to EWU, was the Liberty Lake Park and Ride. Although the trolleybus plan is mainly for the core of the City of Spokane, they at least have a bus route that serves Liberty Lake all day, the 98-Liberty Lake via Sprague Ave. When I was going to EWU, this wa sonly served via an express route on I-90. Although the STA lost a good oppurtunity to have an excuse to serve the Idaho Line when Washington State Lottery did not have Powerball and Idaho did. Definitely would have had a good ridership when the jackpot was high.
As for Interstate Transit between Kootenai County, Idaho and Spokane, WA, I was often thinking of some kind of Diesel Powered Light Railway, if there were still a branchline to Cour d'Lane that had not been abandoned yet, but I doubt that could be possible today. I don't know if the state government in Boise would be of much help. Couer d'Lane only had a bus operation start in 2005. CityLINK