I&#x27;m surprised no one mentioned the already existing (40 years old!)  same project in Paris (called RER). It&#x27;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 (&gt;1 million person&#x2F;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.