Section 7. After compliance with section three and the two preceding sections, and within thirty days after the first publication of notice of the agreement of association therein required, the directors therein named shall apply to the department for a certificate that public convenience and necessity require the construction of a railroad as proposed in such agreement. With such application said directors shall file a map of the railroad showing the cities and towns through which it will pass, the principal highways, railways, railroads, navigable streams and tide waters to be crossed, and the extent to which the route of the railroad will be fixed upon private land or will be located longitudinally upon public ways and places, together with a general profile of the railroad showing the grades, and an estimate showing in reasonable detail the cost of construction. The directors shall also furnish such additional maps and information as the department may require. Prior to the decision of the department the directors may change or modify the route in any city or town in whole or in part either at the suggestion of the department or otherwise. If the department refuses to issue such certificate, no further proceedings shall be had, but the application may be renewed after one year from the date of such refusal.