Let's see, there's a study reported on in Feb. 2011 for Riverside and San Bernardino counties (where one end of the rail line will start initially) that shows that there are only about 64,000 residents who work somewhere outside the neighboring counties (where taking the train would actually be useful). And there's also the comment that some of the supposed long-distance commuters might actually be due to a records problem. For example, even though I lived in San Diego, my parents' address was my permanent address for a lot of things, so I would have shown up as a long-distance commuter due to working so far away from my "home address". A lot of students has this situation as well.

http://beaconecon.com/Misc/RIR_UCRiverside_E1.pdf (page 4)