What I didn't realise about the post office is that there is some mechanical action possible. When the mail flag on the top of the building is pushed down there is a magnet that comes down in the red plastic wall. The magnet can then connect to the mailman's car. When a train passes by on the other side of the post office it will raise the flag, together with the magnet. The magnet disconnects from the car and the mailman's car will run down the ramp. Nice.