The interesting question isn't 'how', it's why.
E.g. one of the 'metro's in the UK is a tram system in Manchester. And the one thing people might call a 'metro', when compared to other cities and countries is the London Underground, which everyone including the government[1] calls The Tube. And that's just a red ring with a blue line through it.
The more I think about it, the more bizarre it makes us Brits sound. Tubes, choobs, toobays...