This is theoretical but perhaps you could modify a short (quarter ?) length track as the finishing line by carefully cutting a centimetre section of track there (two cuts, in one side) and linking the outer segments to restore the ability of electrical current to flow from one end of the segment to the other. You then wrap enough turns of fine insulated wire around a reed-switch to make a current sensitive switch with one end of the wire connected to the outer segments and the other to the isolated centimetre middle sector. The effect of this is that when a car is drawing current from the slot and it enters that isolated sector the current will flow through the coil around the reed-switch (which has now become a reed-relay) activating it which can be used as a volt free pair of contacts (i.e. a proper switch with no built in voltage or biasing issues that can be used anyway you like within reasons).
Of course having the start/finish line like this means that a master switch/relay could isolate all eight of the coils and guarantee an equal start.
If you do not want to mangle a track segment so much you could isolate a whole piece of track in the same way with some isolating material (nail varnish?) on the slot tabs at either end of the segment - hopefully you can still make a connection to a piece of the relevant metal on the underside for one side of the slot.
Note: I have rather assumed that you are using a track made from repeated sections of moulded plastic with metal inserts that provide the conducting rails that the individual cars pick up electricity from.
I have discounted using the magnet from the car's motor to directly activate a reed switch glued to the underside of the track in the vicinity of the start/finish line but it might operate and is in effect a mechanical equivalent to FCML's answer but nothing like as sensitive - and probably too insensitive to work...