First, I've mapped the current subway coming out from spawn. I probably have some mistakes, but the map should be accurate to within 10-15 blocks. The original map I'm working with has a 1 px=5 blocks scale, this one has been scaled up by 200% to make it more readable. I suspect that at some point, I'll make a more easily friendly map, but when developing the subway, I want lots of accuracy.
You should open the map up in a new tab, because it is a bit big for the forums. A direct link is here.
The first thing that needs to be done is to decide on some basic standards for both the rails, and the stops. Everything I say here is tentative, so if you have a better idea, make sure you give it.
For the rails, the tunnel should be at least 3x3 (to allow horses, though 4 high might be preferable), with two lines in them. The powered rails should be placed no less frequently than every 30 meters (28 meters is maximum speed, but when counting based on coords, 30 is a lot easier to keep track of, and only results in negligible slowdown). Including an ice path for sprinting is a nice feature but probably not required. They should be well lit. Decoration is optional.
Stops are defined as points along the rail where the minecart comes to a complete stop. They should be at major locations, so as to prevent people from adding stops to let them access their individual house that slows everyone else down. Stops should be well labeled, and provide easy and free access for the public to go up to the surface and look around. In my opinion, its better to keep stops very simple. I would propose 2 unpowered rails to bring you to a stop, with more powered rails just after them to speed you up. With the current ability to just press 'w' and move decently fast, there is no real reason to go hunting for wherever the button was placed. There is an image below that shows a simple mockup.
I do not know what would be a good design for a rail junction, so give your ideas here. Simple to make is a plus, as is easy to use if you don't know how it works going in to it.
Other things that we need:
I plan on trying to have the rail more or less go to all edges of the map. I'll probably try to plan to keep it close to the centerlines to make it easier to build/plan. However, I need information on what stops need to have connections. Give the coordinates of your stops in this thread.
If there is partial rail somewhere that can possibly be used, give the coordinates of that rail, and I'll check it out and map it.
If you have other ideas, let me know. As I get more information, I will start planning out where we need to start digging our rail. I'm probably also forgetting something from this post, so it will probably have things added to it and continue to grow.