Brillilant picture man, you have an eye for detail... If the car is heavy enough to be able to offset the spring whilst it's in compression I believe you have a working solution.
An idea: Construct a circuit that will sense the car on the rails by either voltage dip (car's contacts connected) or IR. This will then trigger a timer that turns on an electromagnet to hold the car down, ensuring a healthy contact. For this period the car will charge and at the end (eg: 45secs) be released from the magnet, and take off down the track.
You could do this with a 555 timer and a few transistors. You would also need a
custom charger .
ph2t.