Hi VDM,
This is not a stupid question, the car need to run on a track a (-) and a (+) electricity contact. The (+) from the gride roof is collected form the trolley and the pole, and the (-) is a way to produce contact between the floor and the car, previously the contact was done by one of the two rear wheels. Since 1976 this contact is done by a brush : less noice than steel wheel, the brush clean the floor, placed in the middle of rear wheel axis this avoid the car sliding ...
If you have others questions, i will be please to answer them...
Have you got some picture of this car ?