§ 30. General powers of board of transportation. The board of transportation is empowered to operate any railroad acquired, owned, constructed, or provided by such city in accordance with the provisions of law. The board of transportation shall also be empowered to purchase all necessary materials and supplies for such operation and maintenance, and, generally, to exercise all requisite and necessary authority to manage and direct the operation and maintenance of such road for the convenience and safety of the public, provided, however, that, except as otherwise provided in section seventeen of this chapter, any contract for the purchase of such materials and supplies or work or labor to be performed, the estimated cost of which exceeds the sum of twenty-five hundred dollars shall be made by the said board only upon public letting founded on sealed bids, except in a case where the board, by resolution, declares the existence of an emergency or the existence of other circumstances making competitive bidding impracticable. Notice of the invitation for such bidding stating the date and place of the receipt and opening of bids shall be published in the city record in five successive issues immediately preceding such opening. The board may reject all such bids and readvertise for new bids if it shall deem it for the public interest so to do; if not, it shall award the contract to the lowest responsible bidder unless the board, by unanimous vote, shall determine that it is for the public interest that a bid other than that of the lowest responsible bidder shall be accepted.