§ 56. Locomotives must stop at grade crossings. All trains and locomotives on railroads crossing each other at grade shall come to a full stop before crossing, not less than two hundred nor more than eight hundred feet from the crossing, and shall then cross only when the way is clear and upon a signal from a watchman stationed at the crossing. If the corporations cannot agree as to the expense of the watchman, it shall be determined by the commissioner of transportation upon application thereto by either of them. If the corporations disagree as to the precedence of trains the commissioner of transportation may, after hearing, upon the application of either corporation, prescribe rules in relation thereto. The full stop and crossing on signal may be discontinued if the commissioner of transportation shall decide it to be impracticable, or if, with the approval of the commissioner of transportation, an interlocking switch and signal apparatus is adopted and put in operation at such a crossing. The full stop and crossing on signal shall not be required in depot yards, or the approaches thereto, if the crossing roads are under lease or subject to the same management or control in the use of tracks. An engineer, violating the foregoing provisions of this section, or any such rule of the commissioner of transportation, shall be liable to a penalty of one hundred dollars; and any corporation or person operating the railroad, violating any of such provisions or rules, shall be liable to a penalty of five hundred dollars. The commissioner of transportation may, whenever in his judgment the public safety requires the erection of interlocking switch and signal devices at points where steam and street surface railroads intersect at grade, direct the erection of such devices and apportion the expense of construction, operating and maintenance thereof between the companies affected thereby. No railroad corporation, nor any officer, agent or employee thereof, shall stop its cars, horses, or locomotives upon a grade crossing of a railroad of another corporation, for the purpose of receiving or delivering passengers or freight, or other purpose, and any person or corporation violating this provision, shall be liable to a penalty of two hundred and fifty dollars.