Section 7. Every corporation undertaking to perform the kind of public service authorized by this chapter shall thereupon become a common carrier, with all the duties and liabilities of common carriers, and the department shall have the same jurisdiction over the operation and the service rendered that it has over street railway companies, except that no corporation shall be required, without its consent, to extend its trackless trolley service beyond that described in its petition under the preceding section or in its offer to the department thereunder, and the department may permit the suspension or curtailment in whole or in part of the trackless trolley service of any corporation operating under authority of this chapter whenever, by reason of weather or traffic conditions, or of the condition of the highways, or the season of the year, such suspension or curtailment may be desirable for the safety of the traveling public or to avoid loss in operation.