I have uploaded the contents of a booklet to web pages at the link below:

http://www.romilepa.pwp.blueyonder.c...tbk/index.html

It deals with three items of equipment:
Track Circuit Interrupters
Position Detectors
Positive Train Identification

Although this booklet was designed for Northern Line staff, the track
circuit interrupters and position detectors are used on other lines as
well. The position detectors replaced the inductor rails on the Northern
Line in order to be compatible with the 95 stock.


A general description is given of their operation - it's nothing too
technical and may be of some interest. The web pages are adapted from the
original A5 booklet.


Roger