"Bing bong" sings the ticket machine as you swipe your card.
"Bah-da Bah-daa", it's neighbour replies.
"Bong bing!", cries out a third.
"Da-dah", the first retorts.
"Boop boop", "Do do doo", the other machines join the choir and together their mad symphony fills the metro station tunnel.
But what do the lyrics actually mean? I have no idea. There's no apparent pattern, no code. Does each card prompt a different noise?
Most of the noises mean that your card is accepted, so let's just assume that this is a cheerful song, one from the heart of the musical metro station welcoming you through it's ticket machine vocals.