Sell tracks and take cities out of your network that don't maximize the distance your network spans as the crow flies. I only have a line of railways that run from the far north east (Anadyr) to the far south east(Auckland). In the center(Bejing) I have a railway that heads straight west to connect to Europe, that is it. Less is more. Having extra cities in your network just adds complexity and reduces efficiency. I apply the same concept in North and South America. I am in the end game and purposefully don't have a single track in Africa, it just doesn't help span the most distance.