[woodcote]

Thanks Hub! I did spend a few evenings looking at model railway sites and forums to explore the reverse polarity methods they use. The basics are the same, but slot cars do behave differently to trains  - they are much faster and racing cars don't stop at signals! So I shall need to borrow from both model train enthusiasts' and slot car racers' tried-and-tested methods.

Essentially, I will be using standard lap counting devices - dead strip, magnetic reed switches, photosensors or a physical switch in the slot -  to trigger a separate single-channel relay board than will then trigger the reverse polarity DPDT relay board. The important thing is to find something that works with all cars all the time. It might be the manual switch is the only 100% reliable method. I am happy with that (it becomes second nature after a while), but for a public setting an automatic switch will be more important.

However, I'm going to get on and build the track first