(1) The sums so set apart by the Board under sub-section (1) of section 67 and the sums forming part of any sinking fund referred to in sub-section (2) of that section shall be invested in public securities or in such other securities as the State Government may approve in this behalf and shall be held by Board in trust for the purposes of this Act.
(2) The Board may apply the whole or any part of the sums accumulated in any sinking fund in or towards the discharge of the moneys for the repayment of which the fund has been established:
Provided that it pays into the fund in each year and accumulates until the whole of the moneys borrowed are discharged, a sum equivalent to the interest which would have been produced by the sinking fund or the part of the sinking fund so applied.