Glad you like the idea. Thanks for the helpful comments.
Here's a slightly messier version. It's tricky to get the balance right between the need to come up with something which makes sense 'at a glance' and the aesthetic effect that we want (whic,h I agree, needs to mirror some of the sprawl and complexity of a real Tube map, because that is one of the nice entailments of the metaphor when transferred to our plans). I don't think it's quite there yet, but it's getting better. I'll try breaking up the neatness of the green perimeter I think, in the next version.
I'm afraid I can't do broken/dotted lines at the perimeter: I've made the diagram in Prezi, and the template doesn't give me that option. But the advantage of using Prezi is that further down the line we can embed content into the circular frames (the stations) and zoom in to these. So we can use the map to present the proposal, if we get the map to a state where we are happy with it.