A question for Londoners: is it a common belief (as I was told) that stumbling onto the electric rails can be lethal without any train involved?

It's a common belief alright - and it's a perfectly true belief. Don't risk it. Polish tourist killed by urinating on 750-volt electric railway line
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Like most Londoners, I think, I'm just as likely to use the expression "subway" as an alternative to "underground" and "tube", Alan.

As for "pint of bitters", that sounds a highly unlikely drink, even if Emmylou might have sung it.