Section 12. If an electric railroad company, incorporated under this chapter, does not begin the construction of its railroad and expend thereon at least ten per cent of the amount of its original capital stock within two years after the date of its certificate of incorporation, and does not complete and open its railroad for use within four years after said date, its corporate powers and existence shall cease, unless the department, after public notice and a hearing, extends said time by a certificate, stating that it deems due diligence has been exercised by the corporation, and that public necessity and convenience require such extension.