Hail Cart Button
Next we need to wire up the dispenser junction and attach that to a button in the station.
Back on your station level place a lever on the block you want to have your button and start building the wiring underneath. A lever is used at this stage so we can have the circuit activated while we build it. It is much easier to build with an active circuit because you can spot issues as the wire is laid.
This is long circuit and doesn't quite reach where we need it to go.
A repeater will extend the range of the current.
Note in this picture the circuitry hits the dirt block and the power is off after this. Unseen in the picture is a redstone torch extending from the far side of the dirt block. This is to invert the current so that the dispenser junction is activated correctly.
Return to the station level and replace the lever with a button.
Now you can test that the basic layout is functioning correctly.
At this point our station is essentially constructed. So now we need to concentrate on the tracks leading into and out of our station.
After finishing the basic set-up I thought of a couple of improvements which are shown in the above image.
First of all, the Arrival track didn't need to snake back around the way it did. All I needed to do was have the fall into the dispenser with the right orientation. So now the track enters from the front rather than snaking behind.
There is now a piece missing on the Arrival track so the cart stops here before reaching the wooden plate. This gives time to the passenger riding the cart to get off and push it forward. The old way the passenger had to get off fairly quickly before the booster cart would run away with the cart.