I've seen groups of Jeeps headed out on trips in the Freeport - Auburn area. I don't know where they were headed though. I've been getting my fun in the North Maine Woods and other areas near Moosehead. Obviously not local.
Its weird. I refuse to just go on someones land without permission. No old rail road beds here I know of. I constantly see jeeps and trucks all mudded up. They either know something I dont or have terrible drive ways lol
I found out about one local place today. All Wheels rec park in Lewiston. The owner asks for a $10 donation when you go. You can camp for no extra charge. About 500 acres, lots of trails of different types.