The Cart Dispenser
The idea here is that reserve mine-carts sit in the dispenser and are released when required. The return track feeds back into the dispenser.
Hollow out a typical booster channel on the rock 4 pieces long. At the end place a half block. This is where the waiting carts will sit ready to be launched. Returning carts will fall from the top and keep the dispenser full.
I've turned the exit track 90 degrees so the carts in the dispenser have a North South orientation as this is the default orientation for mine-carts.
I accidentally hollowed out the wall between the booster circuit track and the dispenser. You'll see that I patch this back in later with dirt. I should have hollowed out the other side where the coal is - this will be done soon in the instructions.
Now we'll return to the top level and redirect the Arrivals track over the dispenser - but before we do that we need to know the coordinates of the dispenser.
Stand on the dispenser and click the F3 key. This will show full info on the screen including your coordinates. The dispenser is located at 771, 60, 796.
Using the F3 key is extremely useful when trying to work out the position of blocks where you are working blind.
While standing on the dispenser, dig straight up so the hole will be easy to find in the next step.
Return to the top and dig the Arrivals track to drop over the dispenser pointing in the same direction of the exit track. Laying tracks North-South will ensure the orientation of carts in the dispenser will be pointing in the right direction. If your dispenser has an East-West orientation and you log-out, the carts will reposition themselves North-South when you log back in.
Return to the bottom and dig out two more booster channels next to the dispenser. The middle track will be the booster for the dispenser, and the outer track will be to return the booster to its looping circuit.
Leading off from the middle booster track, hollow out more rock and create another standard looping booster. This is the engine that drives the dispenser.
To help with understanding where we now are, the sliver of wood in the image above is the half block we used for the dispenser.
The loop will be powered to be released by a button in the station when we've finished.
You can test the result by powering the junction in the loop.
Now that the dispenser circuit is complete you can load 4 carts into the various booster and dispenser positions on the tracks.