A common carrier by rail may, with the approval of the department but not otherwise, mortgage its present and future property, tangible and intangible, including franchises, to secure the payment of its bonds or notes, including any bonds or notes to be thereafter issued under the provisions of such mortgage; provided, however, that no such approval shall be required for any mortgage of property pursuant to after-acquired clauses of mortgages securing the payment of bonds or notes issued prior to September 1, 1951.