We will see two new Yellowstone shuttles serving the busiest parts of the park in 2021, as National Park Service officials announce two pilot projects for future evaluation.
Traffic has been a huge issue for National Park Service officials in recent years: as visitation goes up (even in 2020, despite COVID-19 concerns): discussions of a shuttle system of some sort predate the Cam Sholly era and go back to the Dan Wenk era, who discussed Yellowstone shuttles as a way to relieve traffic issues.
https://yellowstoneinsider.com/2020/10/21/yellowstone-shuttles-planned-for-2021-debut/