1. Changed the description to "Track Occupancy sensor"
   and added the Reed switch version of the short hall holder.
   This appears to have greater sensitivity than the standard hall,and is not sensitive to the way up of the magnet on the train.

The Reed switch is a RI-80 1.8x5mm type,(supposedly 5-15AT sensitivity) mounted in line with the track for best sensitivity. It will switch when the magnet is a longer distance away than with the hall, but will switch off when the magnet is directly above.

The is is a Hall Sensor mount with a track side cabinet to hold any "local" electronics.
It will help automate a Garden Railway.
The sensor fits standard 45mm Gscale track.
The main photo shows the design with the track-side cabinet, but there is also a simpler version without the track-side cabinet.

I originally used a hall sensor as it should be reliable, although it will need good sealing from rain etc. I have used 25mm circular magnets mounted on the locos with epoxy, but other magnets will work. Be sure to have the hall sensors all in the same orientation, and that all the magnets are correctly oriented to activate the sensors.

I now (2020) suggest using just the Hall mount (or the reed mount) and placing the electronics in an alternate cabinet/ building.
I now use Rocrail "WIO" software and hardware.
(https://wiki.rocrail.net/doku.php?id=arduino:wio-en&s[]=wio)
This video shows the original version running different software.
https://www.youtube.com/watch?v=C6bxuHB6rpU