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I recently watched a video on Youtube where two people use a railway to charge their phone. The person in the video uses a voltmeter and shows there is a small (under 5VAC) voltage difference between the two rails. He attaches a speaker and a lightbulb, with a terminal on each rail, to prove that there is enough usable power. The speaker pulsates a little and the lightbulb does indeed flash. He then uses a diode and a capacitor to rectify and smooth the voltage to charge his phone.

Does this voltage difference actually exist, or is it just some clever editing? What is creating the voltage difference between the two rails?

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    What do you mean by ”standard railway”? Do you mean non-electrified? – gerrit Mar 7 '16 at 16:41
  • Yes. I'm not sure what the proper word is. – Kevin Evans Mar 7 '16 at 16:49
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    Related: en.wikipedia.org/wiki/Track_circuit – iamnotmaynard Mar 7 '16 at 17:16
  • In the US, railways are protected by trespass laws, so one is not allowed to go and see if the rails have a voltage on them. If you interfere with signalling, you are guilty of a crime. – user29285 Mar 9 '16 at 1:47
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What you see in the video is the АЛСН (автоматическая локомотивная сигнализация непрерывного действия) signal, a system for automatic train control used in the ex-Soviet states. Due to the low signalling speed, you can even read the signal from the pulsating bulb, which is here "code Z" (three long pulses separated by short breaks) basically telling a train on the track that the next signal will be green.

Drawing any current from the rails (as done in the video) is very likely to disturb the reception of the signal in nearby trains. A lack of signal on the rails is interpreted by the running stock as if the next signal is "red" and would force nearby trains to stop.

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