Millbrae Station, on the San Francisco Bay Peninsula, has a few problems with its role as a transfer station, partly due to it being just south of SFO airport (and hence, not being between the airport and airport's namesake city), and partly due to how the existing transit operators have treated it.
The two obstacles to Millbrae being a successful HSR/CalTrain/BART/Airport transfer hub are of course the last two, BART and getting to/from the Airport from Millbrae.
BART's trackwork to/from the SFIA-funded airport station is designed to support BART services from both the north and south terminating at the airport; in BART's book, the airport, not Millbrae, is the transfer station for any future BART extension further south from Millbrae. In the absence of such an extension and accompanying increase of south-of-the-airport BART ridership, BART's interest in operating regular and timely connecting services between the airport and Millbrae is minimal, particularly since SamTrans isn't especially keen on paying BART's rates for operating the shuttle.
For travellers then, extending the current SFIA People Mover to Millbrae Station would be beneficial and provide a more consistent and operationally cheaper service than BART. This would also benefit the local hotels especially if the AirTrain was routed around the Millbrae Westin/Clarin Hotels before crossing the 101 Freeway and terminating at Millbrae Station above the bus bays (ie, at the general concourse level). Unfortunately, the cost of extending AirTrain makes this somewhat unlikely, and a compromise involving a regular short (2-car, instead of needlessly hauling around 11 cars) BART shuttle between Millbrae and the airport is more likely.
BART's 3 platforms at Millbrae, while apparently standard for a terminating station on the BART system, constrain the provision of platforms for HSR and CalTrain. There are a number of solutions to this, but the cheapest is for BART to surrender one platform to HSR/Caltrain.
This avoids the expense and service disruption of putting additional platforms underground at Millbrae, but the question is of course, which platform. Platform 3, the one shared with Caltrain, is out for two reasons, one being that it would eliminate cross-platform transfers, the other being that it would place Caltrain/HSR right beside the BART 3rd rail. Likewise, Platform 2 is out, as it has the added complication of essentially separating the BART yard south of Millbrae.
This leaves Platform 1, the eastern-most platform, as being the one which BART should surrender to Caltrain/HSR. In addition to not being beside BART's 3rd rail, it has the benefit of providing a cross-platform transfer between Caltrain/HSR to BART for northbound travel. It does limit BART's future expansion southwards to being a subway (and the swing of the track around the BART yard should probably include a brief supported section/buried tunnel portal to avoid future service disruption should BART do so), but this shouldn't be perceived as a large issue.
Of course, this only provides one of the two extra platforms required. The other platform should come from transforming the western-most platform from a side platform to an island platform, providing cross-platform transfers between Caltrain and HSR in the southbound direction. Access to the station concourse level on the west side would also need to be reworked (probably with an access bridge to the south and west), as a direct expansion to the west would come onto land owned by the Millbrae Serra Hospital.
In addition to running along the boundary of the Hospital, there are certain other issues with this additional track. A corner of the lumber yard to the north would be needed, and the Millbrae Train Museum in the old station building to the south is right in the path. This would either need to be moved closer to the road, or leave it where it is and provide access via the new island platform 5/6.
Alternatively, some rather large checkbooks could be opened and tracks be put underground, but they should be the BART tracks with an extra wide platform beneath 1/2 (current BART) and 3/4 (current BART/Caltrain), leaving the existing platform spaces to be Caltrain/HSR.