I have not been to many in the US, but the best inner park system was in Yosemite California. They had regular trams that complete the valley loop running by stations every 15 minutes.
It would be nice if Yellowstone did that, but I guess the buffalo make it infeasible.
Yellowstone is also 3x the size and has much different terrain and road networks- Yosemite’s transit only covers the valley floor road loop that you use to access all the trails, so it’s only like 5 miles long.
There’s a guy on YouTube who has a series where he travels to different national parks (and other places) via mass transit. It’s pretty fascinating, generally.
Not a national park per se but you can take a bus from downtown Portland, OR to the Columbia River Gorge / Multnomah Falls and Mount Hood!
That along with plentiful hikes in Washington Park makes the area quite accessible for hiking and outdoor activities without a car.
