Section 15. The associates may from time to time, at a meeting called therefor, reduce the amount of the capital stock, but not below the limit prescribed in the preceding section; and they may, in like manner, change the gauge of their railroad to the other gauge allowed by said section. The directors shall appoint a clerk and a treasurer, who shall hold their respective offices until a clerk and a treasurer of the corporation are chosen and qualified in their stead. The directors shall fill any vacancy in their board, or in the office of clerk or treasurer, before the organization of the corporation.