Thanks for the input everyone. It is good to hear the Hiawatha should be OK for our bikes. The section from Mullan to Lookout Pass is still unknown and 1,500 feet of climbing over 11 miles on gravel doesn't sound like much fun but I-90 or bumming a ride are options if necessary.
simplygib - yes we will be camping. We plan to ride from Heyburn State Park to Wallace so won't need to camp in Enanville but we will still check out the Snake Pit. I've read the Wallace RV park now has a brewery so that should be fun! That is part of the reason we may have a bit more difficulty the next day climbing to Lookout Pass!
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Nothing compares to the simple pleasure of a bike ride - JFK