Between Big Sky and West Yellowstone lie a plethora of trails branching into the northwest corner of Yellowstone National Park, two of the most popular being Fawn Pass and Bighorn Pass. Their full lengths span across the crest of the southern Gallatin Range, all the way from Hwy. 191 to the Park's Grand Loop Road, for approximately 20 miles as a through-hike on either trail. These routes will take you from riparian meadows to broad, treeless alpine expanses, and everything in between. You can set up a car shuttle, but note that you'll be driving through the busiest portions of the Park, so expect the drive to take at least two hours.
For a more hassle-free outing, you can start at either end and hike to one of the passes as an out-and-back. Fawn Pass (A) is a bit closer to Hwy. 191, while Bighorn Pass (B) is slightly closer to Grand Loop Road. It's about 2,500 feet of elevation gain no matter how you slice it—a pretty full day given the distance on trail. Another popular, and shorter, option is hiking from the Hwy. 191 trailhead to the Fawn Pass – Bighorn Pass Cutoff (C) and back, for an eight-mile round-trip. Or, you can set up a short car shuttle along Hwy. 191 between the two trailheads, using the cutoff trail for a near-complete 10-mile loop with 1,000 feet of elevation gain.
In the winter, however, you can skip the shuttle and complete this loop by skiing or snowshoeing across the broad flat along the trickling, meandering upper Gallatin River. Just be prepared to break trail, as winds often fill in any established track. In spring, with the right conditions, this area lends itself to excellent crust-cruising. If you're going all the way to Bighorn Pass in the snowy season, beware that the last two miles pass through high-risk avalanche terrain.
Head south on Hwy. 191 past Big Sky. About 9 miles after the sign for Yellowstone National Park, the Fawn Pass trailhead and parking area will be on your left, with Bighorn Pass a couple miles farther. In summer, a short dirt road leads to to a parking area for Bighorn Pass. In winter, park in the pullout right along Hwy. 191.