Generally No, National forests and BLM lands in the southland allow dispersed camping where you can hike/drive in and pick your own spot to camp as long as it is 200 ft away from a water source. Some sections are restricted to day use do to extreme popularity. To find out where to camp, consult the rangers office in the districts you plan to camp in
If it is in Compton, yes.
What you need to do is figure what jurisdiction the train is in. Some are National Forests, some run by the State, some BLM land. They all have different rules. Once you know the location, you can look up their rules online or phone them.