“The provisions of section 703 of the Regional Rail Reorganization Act of 1973 (45 U.S.C. 797span), section 8 of the Milwaukee Railroad Restructuring Act (45 U.S.C. 907), and section 105 of the Rock Island Railroad Transition and Employee Assistance Act (45 U.S.C. 1004) shall not apply to the National Railroad Passenger Corporation in the hiring of qualified train and engine employees who hold seniority rights to work in intercity rail passenger service in connection with the assumption by such Corporation of functions previously performed under contract by other carriers.”