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?