Section 125. If a drawbridge is not furnished with such barriers, and in all cases if by reason of darkness or otherwise the barriers or signals connected with a drawbridge are not visible from the engine of an approaching passenger train, the engineer of such train shall bring it to a full stop at a distance of not less than three hundred nor more than eight hundred feet from the drawbridge, and, before proceeding, shall positively ascertain that the draw is properly closed for the passage of trains; except that if the drawbridge is between two railroad crossings at grade, within six hundred feet of each other, one stop only shall be required for such crossings and drawbridge.