Port districts may acquire privately owned property within such industrial development district and, if necessary, exercise the right of eminent domain in securing the same, in the same manner as in this act provided for the acquisition by port districts of other properties by eminent domain, provided, however, that such right of eminent domain for such industrial development district purposes shall not be exercised as to lands lying further than three-quarters (3/4) of one (1) mile from the water edge of the slack water pool within such port district, created by any downriver dam; provided, further, that notwithstanding any other provisions of the port district law, neither a port district nor an industrial development district shall have power to acquire by eminent domain any existing and operating railroad facilities, without first securing from the public utilities commission a certificate that such acquisition is necessary for the public convenience and necessity.