Except as otherwise provided in this subchapter and in the Railroad Retirement Act of 1974 [
45 U.S.C.
231 et seq.], the insurance programs established by this subchapter shall be administered by the Secretary. The Secretary may perform any of his functions under this subchapter directly, or by contract providing for payment in advance or by way of reimbursement, and in such installments, as the Secretary may deem necessary.