I am finding this safe a little annoying. I come back quite often, sometimes there is a LOT of money there and sometimes there is none. Sometimes it can be an hour or more between visits, yet nothing shows up, other times it its less than 30 mins and money shows up. I cant see any sort of pattern in which the money shows up in the safe of the train. What is the deal with the income for the safe, how often is it added, or at least meant to be added?