Here's a strategy (in the latest version much easier than before) for reference: Add as many carriages as possible.(It also works in New York, Berlin, Istanbul and Auckland). When you're facing a choice between a new line and a carriage, choose the carriage unless you're in urgent need of a new line. Usually 5 lines are enough. You might not even need more tunnels, as 5 tunnels are enough to connect the islands. I got my personal best 1684 and usually 1000 to 1300 using this strategy. Good luck for you!