Now that is a good question, I don't know precisely, but I would guess early 70's. The first set I was given one Christmas was one with a 'rig' and I think a police car. This was pretty much standard fare in terms of track and didn't really have stunt pieces apart maybe from a chicane or similar, definitely no loops or anything. As I recall the loops etc... didn't really come along until the Super G magnatraction cars started to hit the market. There used to be two basic chassis designs, the flat one with a horizontally mounted armature with gearing above it. Then the new super-g cars with the inline armature and much stronger magnets mounted inline also to get best attraction to the track rails. The second set I did get was one with loops and the 'flexi-track' piece, I don't know if you recall that but it was very similar to the multi-jointed flexitrack you got with some plarail setups.
Happily collecting things all my life... 