Gametype:Chaos Theory
From #openttdcoop wiki
The Chaos_Theory Concept
By Gamer, updated by AntB, tneo
The Chaos Theory Concept is a difficult one to understand. Chaos theory is best described by one word: "random." Not much is known when we start building... only the train length and cargoes. Track goes where ever it can go when it's need and lines are expanded as and when they are needed.
Building in Chaos still needs to be clean e.g. no 45 degree turns when avoidable. Chaos is working without a plan, besides the points mentioned above. Whatever you build is up to your good judgement. There is no designated Mainline, as it will grow along the way.
The best way to get an idea of how Chaos Theory works is to download a save and have a look yourself.