The Cartwright Tunnel was originally built in 1912 and 1913 for rail and vehicle access in 1912 and 1913, with the aid of horse- and mule-drawn machinery. It's now part of a short but pleasant pedestrian trail across the Yellowstone River, although as of this writing it's under repair and not yet re-opened to pedestrian traffic. Even a visit to the entrance is impressive, though, and this makes a nice, scenic warm-up for a longer road run in the area.