Start out thinking of DC. Put the [say 48v...] power supply hot (“+”) on the east rail; the return (-) on the west.

Go to the other end of the block, and put a relay across the rails. With no train, and unbroken rails; the relay is powered UNTIL a train axle shorts the rails together; then it drops.

With AC, the only difference is the WeeZbond. It’s a short between the rails at DC, but open at AC. In addition, it’s a DC short to ground; back to the substation.

So the rails form an AC loop, shorted by any axle; but both rails are at DC ground for the third rail power.

The “fluttering” that’s described in news stories is most likely the circuit “ringing” like a tuning fork because of a build-up of energy at it’s natural resonant frequency.

Impedance and capacitance components are chosen to control that behavior in analogue circuits ( like spring and shock absorber combinations (struts) in auto suspensions). The “adjustment” made to compensate for the new impedance bond installed shortly before the accident must have returned the circuit voltages to their static values without anyone realizing that the change that would increase the chance of a resonant condition in some conditions.

The maintenance fix for this is would be to model the resonance of the circuits and to check for “gain” in range of the resonant frequencies stemming from environmental changes and component aging.

The systemic change would be conducting brown bag lunch reviews for system engineers and techs of factors and the need for processes development to minimise the risk stemming from changes in resonant circuit behavior.

The method used, as I understand it, is to essentially use the tracks as a transmission medium for a waveform between… let me check this NTSB document… one of eight “frequencies between 2100 hertz and 3900 hertz.”

For “shunt” to make any sense in this context the only explanation I have is possibly woefully inaccurate and over-simplified. If the signal is transmitted on the tracks as if they were speaker wire, with one the positive and one the negative line. If that was how they were utilized hen shorting them together – shunting – would eliminate their ability to carry the signal.

However as I said, I’m not an electrical engineer, I’m a software guy and my training in signals and actual hardware layer stuff is highly theoretical and sparse.

Electrical engineers sometimes refer to resistors that bypass things like signal circuits and motors as “shunt” resistors, so that’s probably where the odd terminology comes from.