Hi Richard. Scalextric Super Track System (STS) was made by SCX (Exin) in Spain from 1985 to 1990. The off-road 4x4 cars are around 1/43 scale and the track is smaller than the SCX / Scalextric 'Classic' track and not compatible.
The STS 4x4 cars do run on the SCX / Classic track, athough the beauty of STS was the specialist rally-style track system and the smaller size, so you could fit complex layouts in small spaces - maybe a room full of different rally stages.
There are some exceedingly cool videos if you search Scalextric STS or STS 4x4 in YouTube. There's also this great website (in Spanish): https://sts4x4.com/