But Keystone isn’t a public agency, it’s an independent company. So that would be the equivalent of a private highway or say a private parking lot. I’m sure if Trump had won and built the parking lot, he would have let anybody park there.
It's an independent company -- but it operates under public regulation and its services are available to all.
The "common carrier" application of eminent domain goes back to the post roads and ferries of colonial times.