Section 50. Any corporation which owns or operates a railroad, railway, electric railroad or elevated railway shall, upon any increase of its capital stock, except as provided in the following section, offer the new shares proportionately to its stockholders at such price, not less than the par value thereof, as may be determined by its stockholders. The directors upon the approval of such increase, as provided in section forty-eight of chapter one hundred and sixty and section twenty-eight of chapter one hundred and sixty-one, shall give written notice of the increase to each stockholder of record upon the books of the company at a date designated by vote of the directors passed after the approval by the department of such issue, stating the amount of the increase, the number of shares to which, according to the proportionate number of his shares at said date designated by the vote of the directors, he is entitled, the price at which he is entitled to take them, and fixing a time not less than fifteen days after said date designated by vote of the directors within which he may subscribe for such additional stock. Each stockholder may, within the time limited, subscribe for his portion of such stock, to be paid for in cash before the issue of a certificate therefor. No fractions of shares shall be issued, but stockholders may combine them by purchase or sale of rights to subscribe.