There are several problems with this proposal and why it will fail.
Most importantly, what is the incentive for Bitterrooters to use the light rail in the first place? There is really no reason for Bitterrooters to use it as there is nothing even close to gridlock during the morning and evening commutes. One can drive from Hamilton to Missoula doing 65 MPH the entire way over 90% of the time. I also assume that the train would take up to twice as long to make it to Missoula due to stops and speed.
Who will pay for it? Missoulians or Bitterrooters? If the citizens of Missoula should if they don't want the cars. The citizens of the BR valley will never vote a tax increase or bond issue for it.
The system will never be able to support itself if it ever is built. There will never be enough commuter ridership in the valley. It will become a drain on the local governments in Ravalli and Missoula counties.
Just like the Road Diet and Roundabouts, this is all about the local "progressives" wanting to impose European ways of doing things on Western Montana.
It is foolish and doomed for failure.