But things are changing:

https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml?bill_id=201920200SB742

After describing the present situation as of October 8, 2019, the official Legislative Counsel's Digest states:

"This bill would instead authorize the department to provide funding to certain joint powers authorities responsible for the administration of intercity passenger rail services for the purpose of entering into a contract with Amtrak or a public or private motor carrier of passengers for the intercity transportation of passengers by motor carrier over regular routes connecting to intercity rail service, as specified. The bill would authorize motor carrier connections funded pursuant to these provisions to transport passengers who are not connecting to a passenger rail service. The bill would require a joint powers authority that contracts for service pursuant to this authorization, in consultation with the department, to submit a report to the Legislature on or before January 1, 2023, relating to that service. The bill would authorize state agencies and departments, public and private transit operators, intercity motor carriers of passengers, Amtrak, and those joint powers authorities to enter into revenue sharing and ticket selling agreements with each other to provide for intercity transportation of passengers and connections at rail stations to and from local transit systems and intermodal and intercity motor carrier terminals."

It became law on October 8, 2019 after being signed by the governor and filed with the secretary of state.