I say this for your benefit...
Please remember how easily this could backfire. If you are going to charge random kids to use your rails on someone else's property things could go sour very quickly. All it takes is one kid to get hurt, his parents to overreact and the lawsuits flying. If you do an event like this you need to purchase liability insurance, etc.
My suggestion is just keep it to homies and don't let it get too big.