Dover is very fun to ride, but be careful. I know from experience that a couple of neighbors to 2 different entrances like to call the cops when people go ride there. And cops know about the place and occasionally ride by to look for trucks parked near by and wait for them to cross the road while exiting  and get them. And I would strongly advise against going the railroad route. they love to prosecute. Like I said it's a great place, but in my opinion not worth risking the legal repercussions. You make your own decisions, but the better informed you are the better off you are.
I saw a place near USF where I saw a couple quads riding, but I don't know how they got in or if it's legal or not. pm me and I'll give directions, but use them at your own risk. I'd do some research before chancing it. Good luck.