Why exactly is it when you are on the El train that the signal is disrupted? I'm aware that the electromagnetic field generated by the third rail can disrupt the signal, but is it that strong?

wiki says

"All cars on the system utilize 600 volt direct current power delivered through a third rail."

As an add-on, when on the el you're basically surrounded by metal, the car shell/hull thingy, does that factor in? As an another thing-going at good speeds between buildings factor in too :P

When I say "El" I men the elevated trains or the trains on the actual ground. I'm aware that the signal in the subway is blocked due to being ~50 feet underground and in the center of a major metropolitan area.