This is a design I was thinking about submitting to LEGO IDEAS, but it’s just a few specialty parts, not a whole product so this wouldn’t be considered. These are parts for elevators that I would like to see included in LEGO City and LEGO Creator sets one day.
The main part is a sliding door (red) with criss-crossing tracks carved into it. One track is wide and shallow, the other is thin and cuts deeper. This is to ensure 2 different kinds of “trains” (green and purple) can use their own track without accidentally jumping into the other track.
Here it is in action. If you follow the green and purple trains you can see how they interact with the sliding doors, causing them to open and close as the elevator car goes up and down:
This elevator needs a ceiling in order for the trains to be mounted above and below the passenger cab. The one trade-off is that you’ll always have one set of doors open, whichever is currently in front of the cab.
The elevator moves vertically via a string (motorized or manually), lifted by hand, or a corkscrew system. This design allows automatic opening and closing of doors, regardless of how the elevator moves up and down.
I used double doors for the animated example, but it can also be a single door system, to help save some space.
Dave's Geeky Ideas 
really cool