I've been working on a automated minecart system that would allow full carts to be sent to the surface and return empty ones, allowing you to stay down in your mine for extended periods of time.
After a few designs I've come up with a pretty decent one. It allows 6 carts to be sent below. Has automatic tracking of which cart is currently being used so you know how many you have left. It will also switch over to allow the player to return to the surface without getting stuck in the whole mechanism.
*Allow players to return to the surface at any point via a switch in the mine.
*Turn the Remaining carts display to digital instead of an on/off redstone torch
*Setup the lower brake so it can be turned on and off from the surface.
*enclose everything neatly and light it well.
Here are a few screenshots I took. Everything is still pretty open at this point. I'm still working on a few options.