When major construction on the $16 billion Gateway Tunnel gets underway this year, it will eventually double the number of tracks running under the Hudson River between Manhattan and New Jersey. That would, in turn, double the number of trains coming through — except Amtrak says Penn Station can’t fit any more trains. This means when the project is finished in 2038, the new tracks will be little more than a multibillion-dollar redundancy unless something is done. The railroad tried to dodge this question for years, but now that the tunnel is finally funded as of this summer, Amtrak has a plan it wants to sell us.