From a pure detection point of view.

What about using (my favourite detection idea) the piezo cable that is used=
=20
in vehicle speed/counting loggers on roads?

I suspect the track deflects as a train runs over it, so this cable strung=
=20
under the rails would become depressed as the train runs over it causing a=
=20
voltage or pulse change. Someone standing on it wouldn't be able to depress=
=20
it enough, not sure about a herd of cattle crossing the line though.

Cable longevity might be a problem though.

I'm thinking running a fibre optic cable to the tunnel (pulsed or data=20
encoded) would work with the receiver detecting a change in pulse width or=
=20
data information change.  The piezo cable could signal the transmitter to=20
change it's transmission.

Power could possibly be provided by a smallish solar cell with rechargeable=
=20
battery backup.

Colin
--
cdb, colin@btech-online.co.uk on 19/07/2013
=20
Web presence: www.btech-online.co.uk =20
=20
Hosted by:  www.justhost.com.au
=20
=20
This email is to be considered private if addressed to a named  individual=
=20
or Personnel Department, and public if addressed to a blog,  forum or news=
=20
article.
=20
=20
=20

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.