Using codes to call and send minecarts.
This is the minecart storage:
It can hold 16 minecarts in a 4x4 grid of my own minecart cells. The rest of the wiring is two decoders.
The user interface is very simple:
You have to select wether to send a minecart to the selected slot, or recieve a minecart from the selected slot. The code is used to determine which slot is selected. Note that sending a minecart to an already filled slot will cause errors, most likely the minecarts getting stuck. The machine is designed to handle quick movements without errors. If the player fiddles around with the settings while a minecart is being called or being sent will result in either the minecart entering a slot or returning to the player.
Here is a full minecart storage (it's less pointless if you use storage minecarts):