The Attorney General filed no brief and did not care to be heard.

Provision was made by St. 1905, c. 422, for, a new union passenger station in the city of Worcester, in connection with the abolition of certain grade crossings. The entire cost of the land and sixty-five per cent of the cost of the .station were paid by the Boston and Albany Railroad Company, the balance being contributed by the Commonwealth and the city of Worcester. Section 2 of the act reads: “Said station shall be used by all the railroads now entering the city of Worcester, the railroad corporations owning or operating such railroads severally yielding and paying to the Boston and Albany Railroad Company," its lessee, successors or assigns, a reasonable rent *136for the use thereof, which, if not agreed upon by the parties, shall be determined, and may be revised and altered from time to time at intervals of not less than three years, by the board of railroad commissioners upon a petition presented to said board by either of said corporations.” As the parties failed to agree, the Boston and Albany Railroad Company and the New York Central Railroad Company petitioned the public service commission (successor to the board of railroad commissioners) under this statute to determine the reasonable rental for the use of the station to be paid by the Boston and Maine Railroad and the New York, New Haven, and Hartford Railroad Company. After due hearings and investigation the commission made an order fixing the rental, to be effective from January 1, 1916. Thereupon this petition for a writ of certiorari was brought to correct certain alleged errors of law in the proceedings of the commission. A single justice of this court, before whom the case was heard, refused to make the rulings requested by the petitioners, and ordered the petition dismissed.

Most of the items on which the rental was computed are undisputed. The petitioners urge, however, that the commission erred in determining that the thirty-five per cent contributed by the city and State toward the cost of the station should not be made the basis of a fixed charge to be paid by the other railroad companies, and insist that the reasonable rental should be based on the net cost of the present station to the Boston and Albany Railroad Company. Complaint is also made of the view which the commission took on the subject of depreciation. The short answer to all the contentions of the petitioners is that a writ of certiorari lies only to correct errors in law, and not to revise the decision of a question of fact. No question of law was raised at the hearing before the commission. The issue submitted to them, namely, what was a reasonable rental for the use of the station, was within their jurisdiction, their finding was within the scope of the petition, and was a finding of fact.' They prefaced their order by a preliminary statement containing a history of the proceedings, and the reasons which led them to a conclusion which they thought equitable between the parties. Without intimating that the statement fails to fully support the order, it seems plain that it contains only findings of fact. It *137certainly contains no ruling of law which renders the order itself invalid.

Our only doubt arises from the action of the commission in making the award retroactive to January 1, 1916. See Boston & Worcester Railroad v. Western Railroad, 14 Gray, 253, 265. But we think they did not exceed their authority, in view of their express finding that “The parties have agreed that the award so made by the commission shall be effective from January 1, 1916.” While the letter of counsel for the petitioners, dated September 19, 1918, indicates that this statement was not entirely in accordance with his understanding of the agreement, we must accept the finding of the commission as final, for the purposes of this petition. Farmington River Water Power Co. v. County Commissioners, 112 Mass. 206. Haven v. County Commissioners, 155 Mass. 467.

Exceptions overruled.