I read about this stuff in Popular Mechanics. It's called the Bare Conductive Electric Paint Pen. It can transmit up to 12v DC. You draw the circuit and wait for it to dry. It is water soluble and can be removed. If sealed with a waterproof paint it can last a year.
One of the difficulties I have had with adding lights to a slot car is finding room to stuff the wires. If this stuff can be connected to the LEDs and the power source it might make adding lights to a slot car a breeze.
I have ordered a pen and after I try it I will report back.
Here is a link.
http://www.karlssonrobotics.com/cart/ba ... 7AodWAoAyQ