As soon as you have connected your RPi with the PiFace and installed the recommended software (python-libs and piface-emulator) you may gain control over a slot car step by step. All you need to begin besides a simple slot car track, the RPi and the PiFace are two isolated wires of approx. 1 m length in different colours and perhaps an extra control unit, since you have to disassemble one.
In order to support the elinux.org-community further details were published here:
Und hier gibt’s eine erweiterte Fassung auf deutsch: