OK, I'll bite. I used to live in Simi Valley and work in Chatsworth, CA. About a 12 mile ride. In Simi Valley it was about 3 miles to the only commuter station in town. In Chatsworth it was about 1 mile by track, and 2 miles by road between my work and the only commuter station in town. On the Simi Valley end it was about a one mile walk and one bus to the station. On the Chatsworth end it was about 1/2 mile walk and 2 buses (with a transfer).
Sure, the train ticket allowed you on the bus for free. But since they set up Simi and Chatsworth in two different zones, the ticket was something like $20 a day (this one I am not sure about. I do remember doing the math and finding out it was cheaper to buy the car, gas and insurance).
Is this what you had in mind by "meaningful public transportation?" If so, sorry, no go. If not, then almost all of southern CA is out.
But that gets to the point of commuter rail. It's intended to get you from a low or medium density area (your home) to a high density area (your place of work). It's intended to bypass a congested highway system by using its own guideway. You may have to take a bus or drive to the local commuter rail station, but that's normal if you don't live next door to it.
But the greater issue is whether a commuter rail train gives you an advantage over driving, either with respect to time, congestion or sanity. Your Southern California commute would not work very well by train. Neither did my SoCal commute.