hitchhiking in america is perfectly safe, provided you don't do anything stupid. Anyone that tells you otherwise is a paranoid loser who doesn't know jack shit about the american dream or common decency.
i have and always will pick up hitchhikers, provided i have space to stop and you aren't aggressively dirty or obviously intoxicated.
trainhopping is not worth it. I could write you a lot of words on why it isn't worth it, but it's easier if i say that if you want to have an adventure, there are better ways to do it with a substantially lower chance of assault or imprisonment or death.