tunnel and the methods of constructing such works are discussed in the article on Tunnels.
A switchback is a line which zigzags back and forth along the side of a mountain and thus gradually climbs to the summit level at which a direct crossing is possible. These structures are expensive to operate, because of their length and steep grades, and railway managers usually substitute tunnels for them as soon as the finances of the road and the amount of its traffic will warrant so costly an undertaking. Another method of overcoming steep mountain grades is to use a rack railway or a cable incline railway, and these special forms of road are described in succeeding paragraphs.
Track Construction. The width of the roadbed at the top is from 20 to 32 feet for double track and from 14 to 18 feet for single track on embankment. In cuts the width of roadbed exclusive of ditches is from 28 to 33 feet for double track and from 18 to 22 feet for single track. The surface of the roadbed at subgrade is almost invariably crowned at the middle so as to drain off water to the sides. On the top of this crowned surface is constructed the track. This consists of the ballast, the ties, the rails, and their accessories. Ballast is used for four principal purposes: (1) to distribute the load over the roadbed; (2) to form a support for the ties; (3) to provide efficient drainage under and around the ties; and (4) to allow of surfacing and arranging the track without disturbing the roadbed. At this point it is a matter of some interest to note that the term ballast originated in England when gravel ballast was taken from ships for building tramroads. The materials most generally used for ballast are broken stone, furnace slag, burnt clay, gravel, sand, cinders, and earth, but other materials, as shells and chert, are often used locally. These materials rank in merit about in the order named, but the gravel is the material most used in America and after this comes broken stone. The ballast is usually level with the tops of the ties and about one foot thick, and it is usually shouldered out beyond their ends.
TYPICAL SECTIONS OF RAILWAY ROADBED AND TRACKS.
Ties, or cross-ties, as they are often termed, are the transverse wooden sills to which the rails are attached. As stated above, they are imbedded in the ballast. Wood is the almost universal material for ties in the United States, but in other countries metal ties are quite extensively employed. About 55 per cent. of all the ties in the United States are of oak and 22 per cent. are of pine. The remaining 33 per cent. is divided between cedar, chestnut, hemlock, cypress, and other woods. White oak is considered the best wood for railway ties, and ties of this wood have a life of about eight years. Ties are generally from 7 to 10 inches wide, 6 inches thick, and 8 to 9 feet long, and they are spaced from 18 inches to 2 feet apart in the track. An immense amount of timber is consumed annually for railway ties, as a brief estimate will demonstrate. Assuming that 2500 ties per mile of track are employed on the average, then the 250,000 miles of railway track in the United States require 625,000,000 ties. The annual consumption is about 76,000,000 ties for renewals and 14,000,000 ties for new construction, a total of 90,000,000 ties or nearly 300,000,000 cubic feet of timber. In view of these figures, it is not surprising that railway managers are finding greater diliiculty each year in securing ties, and that they should be resorting to measures which will cut down the consumption.
TIE PLATE.
One of these measures is to plant trees, but the one most commonly sought is to treat the ties used by some of the various methods for preventing or rather for delaying the natural decay. Another resort for prolonging the life of ties is to use tie plates, which are plates of iron inserted between the top of the tie and the bottom of the rail so as to distribute the load over a larger area and thus reduce the tendency of the rail to cut into the top of the tie. By many engineers it is thought that the ultimate solution of the problem will be the substitution of metal ties. An endless number of forms of metal ties have been tried, but only a few of them have proved successful, although these successful forms have given excellent results in many instances. In Europe, India, Africa, South America, and Mexico, metal ties are extensively used.
STEEL CROSS-TIE.
Rails are now universally made of steel and are of the flanged T-section invented by Col. R. L. Stevens in 1830. This statement is wholly true of American practice and is generally true of foreign practice. Practically the only exception to the flanged T-section is the so-called bull-headed rail used in England. This rail has the
