Because trains aren’t economically viable for the vast majority of the US, and where they are economically they are the topic of conversation.
As far as why the conversation would center around the US, that’s just the regular American-centric tilt english conversations generally lean towards. Most of Europe has their shit together in some topics like this (public transportation, for instance) and the US is a huge consumer of automobiles and no one if building mass transit between the middle of nowhere to the other middle of nowhere where we could ‘efficiently’ move individually insignificant numbers of people at a time.