Page:The Working and Management of an English Railway.djvu/69

 smaller the radius—that is the sharper the curve—and the higher the speed of the trains, the greater must be the super-elevation. In practice it must be adjusted to the average speed of the fastest trains that are likely to run over it—that is, the express passenger trains.

Railway curves are invariably arcs of circles of greater or less radius, except for a short length, where they join or leave the straight line. Where the radius of the curve is uniform, a uniform super-elevation is given to the outer rail, but this elevation dies away gradually to the straight line, where the rails become level. As the super-elevation decreases the radius of the curve increases until it becomes infinite, or, in other words, until the straight line is reached.

If it be asked what is the steepest gradient a railway should have, the answer is, "The best that can be obtained under all the circumstances; or, in other words, the nearest to a dead level." There are cases in which the physical conditions of a country will only permit of a railway being constructed with sharp curves and steep gradients, except at a practically prohibitive cost. It is, nevertheless, a fact that the less the deviation from a straight line and moderate gradients, the greater is the safety and economy of working; and, in constructing a main line of railway, it is frequently worth while to incur heavy outlay at the outset rather than handicap the undertaking with difficulties of working, which will place it at a disadvantage in competing with other railways for the same class of traffic, for which punctuality and speed are of the first importance.

In practice it is usually found that sharp and frequent curves are associated with steep gradients, and, upon