> Amtrak and Acela do not actually own the railway they use...

This is true for most of the Amtrak mileage, but the Northeast Corridor rails are largely owned by Amtrak[0,pg4]:

Amtrak owns and operates 363 route-miles of the 457-route-mile Northeast Corridor (NEC) spine between Washington and Boston.

[0] https://www.amtrak.com/content/dam/projects/dotcom/english/p...