Bridge builders, as a rule, build upon a tangent, and are very particular to have the floor upon a dead level. The reason that these two conditions are thought necessary is to avoid too much strain upon the structure, and in building railroads they are generally looked upon as absolutely necessary. Where the conditions are such as to admit of such construction it is undoubtedly the part of wisdom to follow the beaten path, but occasionally such a course would either largely increase the expense, or, as in the construction of the Mount Lowe Railway, stop further progress. Such a dilemma was thrice presented in the construction of the Mount Lowe Railway. First in the numerous bridges along the lower portion of the route, in Rubio Canyon; again in building the Great Cable Incline, and lastly on the Alpine division, where it became necessary to build a circular bridge in order to get a proper grade.

In the second instance the bridges were built on the same incline as the balance of the grade, in one instance the upper end being one hundred feet higher than the lower in a length of 200 feet. It was, however, in the construction of the circular bridge that the most radical departure from the usual rules of bridge construction was taken. At that point it was necessary for the track to swing around a spur of the mountain, making a circle about 400 feet, with a diameter of 150 feet across and on one side a deep canyon had to be bridged. This was accomplished by the construction of a circular bridge built on a grade of 4-1/2 per cent. Many engineers would have declared that a car could not be run over such a structure, but it was so carefully and scientifically built that cars are run over it with as much ease and safety as over any other portion of the road. Stoppages are often made upon it and the start is again made without any strain upon either the bridge or car.

On other portions of the Alpine divisions loops are made around the heads of great canyons, and the track turns upon itself in such a maze that in one place nine different tracks can be counted on the mountain side, each successively reaching a higher altitude, all the bridges along the line conforming to the curvings and twistings of the track.