BCL
Engineer
There used to be a free shuttle from Millbrae Caltrain. Also, the BART station at the airport was built into the international terminal design. At the time, the City and County of San Francisco insisted on BART going directly to the airport.In my planners opinion (in an ideal word). That wouldn't have been a Bart extension to the airport but SFO Airtrain would have been extended to Milbrae where all Bart trains would have terminated. Would have made better connecting to everything. The Bart station only serves the international terminal so from many flights your directed to the SFO Airtrain already.
Caltrain riders shouldn't have to transfer trains and modes up to 3 times from Caltrain to their terminal
http://www.sfgate.com/news/article/It-s-up-up-and-away-for-BART-SFO-extension-3089755.php