Joe,
The Passenger Rail Investment and Improvement Act of 2008 provides the direction on this.
Passenger Rail Investment and Improvement Act 2008
The act has specific language regarding "short distance" intercity corridors (750 miles or less). This 2008 Act language that "codified" the local JPA takeovers of LOSSAN, the San Joaquin's, and Capitol's.
The act is "fuzzier" and less clear on long distance corridors, though theoretically doesn't prohibit a long distance Joint Powers Authority. These would be politically very difficult with the multi state political and financial support they might require.