The sleepers, as was said, are laid in ballast, which should as far as possible consist of gravelly sand, sufficiently permeable by water to allow the rain to pass rapidly into the surface, and to the side drains. \Then, however, ballast of this description cannot be obtained, it may be replaced by sand, by shingle, by broken stones of not more than 2i inches in diameter, by burnt clay, or by a mixture of chalk and flints ; in mining districts forge ashes and broken elag are advantageously used. The ballast is made (for double roads) 30 feet wide and 18 Inches thick, on the average ; and its surface is dressed off so as to form a series of water-courses. It requires about 8,800 yards cube of ballast per mile forward of railways, or about 5 yards cube per yard lineal.
The form of rail used, and its weight, have passed through many changes, but they have finally resolved themselves into two principal varieties, known as the double-headed rail, and the bridge rail ; the latter being employed on lines such as the Great Western, where continuous longitudinal bearings have been adopted instead of the ordinary chairs end sleepers. The advantages and disadvantages of the bridge rail will be discussed hereafter, and for the present, attention will simply be called to the steps which have led to the adoption of the double-headed rail moat frequently employed.
In figure 11, the illustration represents the section of the fish-bellied rail used on the Manchester and Liverpool line; I, Is the same rail fixed in its chair; c and d are representations of a mil of the same form used on the Newcastle and Carlisle railway ; these rails were all fastened with iron keys. On the London and Birmingham railway Robert Stephenson introduced a modification of this system represented by c and f; this rail was partially replaced on the line of railway just mentioned by the form shown on 71; and at i is shown a contrivance for keeping the rail in its seat. The double-headed rail may be considered to have been introduced by Mr. Locke, who used it on the Grand Junction and the South Western lines in the form shown at k; and eventually, notwithstanding the other modifications of the rail with intermediate supports as at ns, Mr. Locke's form has been generally adopted. At g ,u, and o, are represented the manners of forming the end joints of the rails, either with a half lap, or with butt ends, the system now prevalent ; and at l is shown a rail of a peculiar section designed to fit more closely to the tires of the wheel. At the
present day, however, little attention is paid to this theoretical perfection of the roadway, and the upper and lower surfaces of the rails are made identically the same. When firstly the double-headed rails were introduced they were made in 15 or 16 feet lengths, of about 56 lbs. weight per yard forward; at the present day they are made of from 70 to 80 lbs. per yard, and are often of 21 feet length. The cast iron chairs have also been increased in weight, and instead of weighing for the joint chairs 20 lbs., and for the middle 15 lbs. each, they now weigh for the joint chairs 38 lbs., and for the middles 21+ lbs. each. Latterly when the rails have been fish-jointed, or made with fish plates weighing about 28 lbs. per pair bolted on the respective sides of the rail, the middle chairs only are employed, and the sleepers at the end of each rail are brought as closely to the fish joint as possible.
When Mr. Brunel attempted to introduce the broad gauge he also attempted to introduce the American system of placing the rail with a continuous bearing, and thus was led to adopt the form of rails represented by p, q, r, and a on fig. 12, partly with the hope of being able to diminish the weight of iron. He began to use rails weighing about 45 to 47 lbs. per yard, and they were bolted or spoked to the longitudinal baulks at distances of about 18 inches ; but the continually increasing weight of the locomotives, and the increased rate of travelling, have forced the parties who have used the bridge rail to follow the example of the advocates of the double-headed one ; so that at the present day the bridge rails are often made of even 90 lba. weight per yard forward. The great objection to their use lies, however, in the necessity they involve for a very expensive timber substructure, which is liable to rapid decay, and to frequent deformations by the warping of the wood, or by the expansion of the rails. Practically, then, although the system of continuous bearings produces a road of a very pleasant description for the traveller, so long at least as it is in order, It is found to be of so costly a maintenance that it is rarely used in England, unless when the roadway has to be carried over viaducts, bridges, or other works, where the percussion upon the intermediate bearings might become injurious.