Ultrasonic and Infra-red detection of train direction

[gregh]

If you still want to sue the ultrasound, could you mount them from an overhead gantry. I say them, because I agree with the previous poster suggesting to use two sensors, spaced at a known distance.
In that regard, could you scrap the US and put two Hall Effect sensors in the track spaced at a known distance. Same math as using Reeds, just that Hall Effects are solid state and less expensive.
You might need to put three sensors, placed at known distances, in order to achieve the logic to detect the entrance and exit scenarios you described.

1. I can't see that pointing a US detector downwards would be any different to across the track , except keeping the rain out..
2.Part of the original 'specification' is "no modifications to trains". ie any visitor must be able to run. So no putting magnets on trains.
3.Others have also mentioned using 3 detectors. I reckon I only need two. Can anyone explain why 3 are needed?

Thanks for the interest.