Subject to the limitations prescribed by section 179.1 a savings bank may invest in equipment obligations or certificates secured by (a) a railroad equipment or car trust, or (b) a lease or conditional sale of or security interest in, or (c) a lien on, railroad equipment; provided, that a savings bank shall not invest in any such obligations or certificates:
(1) if, as of the date of issue, the original aggregate principal amount of all obligations or certificates of the same issue exceeded or exceeds (i) 80% of the lesser of the following amounts: (x) the aggregate purchase price originally paid, or to be paid, for the equipment securing the same, less the aggregate depreciation (if any) accrued thereon to the date of issue, and (y) the aggregate estimated replacement cost of such equipment as of the date of issue, less the aggregate depreciation (if any) accrued thereon to the date of issue, or (ii), if either amount is not ascertainable, 80% of the other amount; or
(2) unless all obligations or certificates of the same issue mature within 15 years from the date of issue, in equal installments beginning not later than 3 years after the date of issue and payable at intervals of not exceeding 1 year.