The Metrolink trains have "bike cars". On the lower level of the car there are seats on just one side, with the other side of the car open for bikes in 4 sections separated by a bar. Each of these cars can accommodate maybe 16 bikes parked 4x4, so unless the car is already full you should not have a problem. You simply walk aboard the train rolling your bike with you. Park the bike to one side, fasten one of the car's retaining straps around the top tube and then sit down across the aisle from it. I believe you can catch the Metrolink as far south as Oceanside and you can get a direct train to San Berdoo as far south as Laguna Niguel, so you have some options on your train/ride mix. Don't need advance reservations on Metrolink, you do on Amtrak to bring a bike.
Camping on various "public lands" in SoCal? Never seen it and most parks are posted no overnight. Unless you are truly homeless and camp under bridges then they leave you alone. But you might not enjoy the company there. Good luck.