After roughly 2/3 of the way east up the Columbia River Gorge, Amtrak's Empire Builder heads north to Spokane then across northern ID, around Glacier National Park and then across the "High Line" (the U.S. 2 corridor) in MT and then across northern ND before dipping down to Fargo. You could have up to a couple hundred kilometers between you and rail line. Many more if you take the more southerly option of the L&C route that passes through places like Wisdom, Dillon and Billings. (For example, Missoula to Whitefish to catch the train would require a good 2 days of riding, if not 2 1/2.) Also keep in mind that you can only get on board with a bike at stations with checked baggage service. (You will also need to box your bike.) In MT, the only stations with checked baggage service are Whitefish, Shelby and Havre. In ND, you have only Williston, Grand Forks and Fargo.

If you are planning to go the southerly way across Lost Trail and Chief Jospeh Passes and down into Dillion, etc., I would expect those "just in case" moments to be "more likely than not" moments. Chief Joseph is around 7,300', and Wisdom, in the valley, is right around 6,000', which is why the average low in October is about 20. When we were there in '11 it got down to about 39 the last night of June. The nice thing is that if it's clear and sunny that helps take some of the chill off.

Not trying to discourage you at all. I am sure it will be very pretty. Just want to let you know what you might encounter. BTW...If you find yourself in Twin Bridges, make sure to check out the Bike Camp there. It's a very nice facility.