In message ,
TKM writes
Well, you've probably figured out that your steel tape completed a
circuit between the two rails within a "block" (an electrically
isolated stretch of rails that controls a set of signals). The circuit
is powered by batteries, but I've forgotten the battery voltage.
However, it's not enough to cause electrocution should you straddle the
rails with bare feet. If you look at the rail joints, you will see a
braided wire fastened to each side so the circuit is continuous and
reliable. The system has been used for many years and is standard in
the U.S. Today, there's probably a line to a computer somewhere that
indicates which blocks are active (indicating the presence of a train
or a fault. Maybe that's what brought the railroad truck and its
hostile occupant to check you out.

Now I'm getting evil thoughts about a little black box with two flying
leads that powers itself from the sense voltage and obligingly provides
a quick shunt every hour or so.

Hmm, a bridge rectifier for polarity independence, a darlington for the
shunt and a PIC12 for simplicity.

--
Clive Mitchell
http:/www.bigclive.com