Underground, perhaps (although German's U-Bahn is the equivalent). Metro, accent aside, is in common use worldwide, isn't it? (Tokyo, Washington DC, to name two Metro systems)
When people say Metro I think the Mexico City Metro. "Metro" is not only used to brand the trains, but also the BRT service ("Metrobús").
When people mention The Tube I do think London though. I haven't heard anyone else call their subway "the tube". To be fair though, the London Underground _is_ quite more tubular than other systems around the world.
When people mention The Tube I do think London though. I haven't heard anyone else call their subway "the tube". To be fair though, the London Underground _is_ quite more tubular than other systems around the world.