From Mullan and the Trail of the Coeur d'Alenes, you can take a short ride out Larson Road to Shoshone Park and Hale Fish Hatchery and this trailhead. This trail follows the old Northern Pacific Railway and has easy climbs with unique RR features including a switchback and a tunnel.
Unfortunately, the trestles are long gone from this section but there are newer road sections to get you back on track at each valley crossing. There are restrooms at Willow Creek and at Lookout Pass Ski Resort. When you make it to the ski resort, you are at the summit and ready to cross into Montana. There is a bike shop and food at the summit, too.