Though the Union Pacific is one of the West's largest railroads, it barely enters Montana ... just a line from Idaho up to Silver Bow, and a now-abandoned branch into West Yellowstone. The latter line was only open in the summertime, and was built specifically to bring tourists to Yellowstone Park. It was by far the most popular rail route to Yellowstone.
For close to 40 years, the UP promoted its Yellowstone service with a series of clever, annual drawings featuring cartoon-like bear cubs, mostly drawn by an Omaha artist named Walter Oehlre. The theme was continued in other promotional materials, as well ... things like luggage stickers and children's' dining-car menus. It was definitely one of the most appealing of railroad advertising programs.
I'm trying to locate a digitized copy of one of the Oehrle ads, but they're surprisingly hard to find. Here's an ad that predates that era, but shows the beginnings of the theme. It's from 1912, and reveals that, at least for a little while, someone in the UP's advertising department thought that West Yellowstone was in Wyoming!