Overground trains are designed for customer safety, in the sense that they have no doors between carriages. This leave less likelihood for you to find yourself almost alone in a carriage with a nutter or gang of steamers as there is very likely to be other passengers within shouting distance and this deters criminals and nutters. More importantly if you don't like the look/smell of the person who is sitting too close to you, it is easy to move down the train to where other passengers are located.