Mass. Gen. Laws ch. 161, § 122

The company shall provide an improvement fund of an amount required by the department but not exceeding five per cent of the capital investment. Such fund may be raised by the issue of bonds or stock or preferred stock. It shall be fully subscribed within sixty days after determination of the capital investment of the company, and shall be paid in from time to time, as required, by a plan of gradual improvement to be approved by the department.