Section 69. A railroad corporation may aid in the construction of any branch or connecting railroad within the limits of the commonwealth, whether connecting by a railroad or steamboat line, by subscribing for shares of stock in such corporation, or by taking its notes or bonds secured by mortgage or otherwise, and may vote on all shares of stock so subscribed for and held; but a corporation shall not so subscribe to an amount in excess of two per cent of its paid-up capital stock, or mortgage its property to secure the loans or subscriptions made by any other corporation under this section, except by a vote of a majority in interest of the stockholders at a meeting called therefor.