PROVIDENCE, R.I. — The plan to build a new Amtrak train station at Rhode Island's main airport could cost up to $90 million, according to a joint study from the railroad and the state Transportation Department.In addition to the capital costs, Amtrak trains stopping at T.F. Green Airport in Warwick would make every non-Acela trip from New York to Boston longer.The study was initiated by Democratic U.S.