The trains were gone by the time my father was a child and he wandered as far as a bike could take him and back in a day. He would play at Army Dumps that dotted the landscape in the 50's.
When I was a kid the freeways and major thoroughfares created islands that couldn't be crossed by a kid on a bike. The farmland and riparian areas around creeks created walking corridors and I would spend time watching tadpoles and building forts.
For my kids the farmland was developed and the creeks tightly fenced off by an interlocking thicket of HOAs. They wandered virtual worlds.