I'm surprised no one mentioned the already existing (40 years old!) same project in Paris (called RER). It's full scaled train (even double decker) running underground in Paris and overground outside. The oldest line (A) is the most used line in the world expect Japan (>1 million person/worked day). Auber station was (is?) the biggest underground station in the world and Chatelet-Les Halles is the most used underground station in the world.