Why was Chris thrown in jail on his way from Seattle to California?

Chris was thrown is jail for hopping freight trains, which is illegal.

“Ron,” McCandless laughed, “I’m not in Seattle. I’m in California, just up the road from you, in Coachella.” Unable to find work in the rainy Northwest, McCandless had hopped a series of freight trains back to the desert. In Colton, California, he was discovered by another bull and thrown in jail.