In order to enter the park during the winter season, you either have to be on a snowmobile or your have to pay to ride a snowbus.
Our snowbus was scheduled to leave West Yellowstone in the morning (we stayed there the night before). However, as we were starting to pack up (we had to drive about 4 blocks to where the snowbus was going to meet us), Grizz locked the keys inside the vehicle... while it was running! Thankfully, there was a repair shop next to our hotel, and Grizz was able to borrow a screwdriver and a flexible metal rod to break into our vehicle. Long story short, we still made it to the snowbus before it was scheduled to leave!
Yellowstone Post #1
Yellowstone Post #3