You don't need to replace Caltrain to achieve that. A easier solution would be to convert Caltrain to a gated fare system (like BART is now), and then put a combined BART and Caltrain station behind one set of turnstiles at Millbrae.

While we're at it, why the hell isn't there all-day, every-day direct service from Millbrae to SFO? Having a massive multi-transit-agency station right by the airport, only to tell people "nope, can't get there from here, you have to go to San Bruno and change trains" is just dumb.

Every day direct BART service between SFO and Millbrae was the original intent, and they provided that level of service when the station opened. But ridership fell well below expectations, and BART eventually limited that service to nights and weekends only.