The railroad company, or the corporation, partnership or person operating one or more steam locomotives under the provisions of section 72 of the Railroad Law will be held responsible for the general design and construction of the locomotive boilers or steam reservoirs on fireless steam locomotives under its control. The safe working pressure for each locomotive boiler or steam reservoir shall be fixed by the chief mechanical officer of the operator or by a competent mechanical engineer under his supervision after full consideration has been given to the general design, workmanship, age, and condition of the boiler or steam reservoir, and shall be determined by the minimum thickness of the shell plates, the lowest tensile strength of the plates, the efficiency of the longitudinal joint, the inside diameter of the course, and the lowest factor of safety allowed.