The location of the line of a railway through difficult country requires the trained judgment of an engineer of special experience, and the most difficult country is not by any means that which might at first be supposed. A line through a narrow pass almost locates itself. But the approach to a summit through rolling country is often a serious problem. The rate of grade must be kept as light as possible, and must never exceed the prescribed maximum. The cuttings and the embankments must be as shallow as they can be made--the quantities of material taken from the excavations should be just about enough to make adjacent embankments. The curves must be few and of light radius--never exceeding an arranged limit. The line must always be kept as direct as these considerations will allow--so that the final location will give the shortest practicable economical distance from point to point. Many a mile of railway over which we travel now at the highest speed has been a weary problem to the engineer of location, and he has often accomplished a really greater success by securing a line which seems to closely fit the country over which it runs without marking itself sharply upon nature's moulding, than if he had with apparent boldness cut deep into the hills and raised embankments and viaducts high over lowlands and valleys.
[Illustration: View Down the Blue from Rocky Point, Denver, South Park and Pacific Railroad; showing successive tiers of railway.]
But roads must run through many regions where very different measures must be taken to secure a location practicable for traffic. For instance, a line at a high elevation approaches a wide valley which it must cross. The rate of descent is fixed by the established maximum grade, and the sides of the valley are much steeper than that rate. Then the engineer must gain distance--that is to say, he must make the line long enough to overcome the vertical height. This can often be accomplished by carrying it up the valley on one side and down on the other. Tributary valleys can be made use of if necessary, and the desired crossing thus accomplished. But at times even these expedients will not suffice. Then the line is made to bend upon itself and wind down the hillside upon benches cut into the earth, or rock, curving at points where nature affords any sort of opportunity, and reaching the valley at last in long convolutions like the path of a great serpent on the mountain side. These lines often show several tiers of railway, one directly above the other, as may be seen in the illustrations on pages 49 and 51.