KathyWdrf

Amtrak is NOT cheap. One way SF / LA nondiscounted price, at a minimum, is $59 to $61 (but rates can run much higher depending on departure time/date -- as high as double for the same coach class seat). Discounts are possible, e.g., AAA 10%, Senior 15%, etc. So, figure a minimum of $100+ for a roundtrip WITH discount.

Also, the Amtrak schedule is VERY sparse between SF and LA -- there is really only one through train per day, but even that one is from Oakland to LA Union Station, so the Amtrak connector bus from SF (or alternative transport) needs to be taken to Oakland first. Then, that Oakland - LA "Coast Starlight" train takes over 12 hours -- just the train ride itself!

The other Amtrak "train" trips between SF and LA actually involve very lengthy connector bus trips on one or both ends, connecting to intermediate points such as: Bakersfield, San Luis Obispo, Santa Barbara, etc. So you'll actually spend many hours on a bus, but pay about the same "train" fare for each itinerary.

So, don't assume that Amtrak in California is anything like the trains in the US Northeast Corridor -- the California service overall is pretty pitiful, to put it mildly. (There are some useful train services, though, such as the Capitol Corridor trains between the Bay Area and Sacramento. But Sacramento isn't where the OP is planning to go.)

However, the Amtrak service between LA and San Diego isn't too bad -- it's called the Pacific Surfliner and runs several times per day. Takes roughly 3 hours, costs about $37 one way for coach class, $56 one way business class. So, decent service (in terms of frequency) but again, NOT cheap.