| Distance between insides of ties being | |||||
|---|---|---|---|---|---|
| 3′ | 3′ 9″ | 4′ | 5′ | 6′ | |
| Weight in lbs. per yard. | 50 | 59 | 61 | 67 | 79 |
| Depth of rail in inches. | 4½ | 4⅝ | 4¾ | 5 | 55 16 |
At the time these dimensions were given, however, much less weight was applied to the rails than at the present day. As the bearing is increased, the rail must become heavier and more expensive; but the number and cost of the ties is lessened. The report above referred to, concludes that five feet bearings, involving heavier rails, would cost no more, after the road bed is consolidated, than shorter ones; but that on embankments and soft subsoils, it would be at first somewhat more expensive.
285. The object of the ballast is, first, to transfer the applied load over a large surface; second, to hold the timber-work in place, horizontally; and third, to carry away the rain water from the superstructure; it also furnishes the means of adjusting the timber-work to the proper position. It should be at least one half way up the depth of the tie, and deep enough below the under surface to prevent the timber being forced down by the passing weight. From various observations it appears that there should be one and a half times the depth of the tie of ballast, beneath the under surface; or the whole depth of ballast should be from two to two and one half times the depth of tie.
For ballast, broken stone, gravel, or other dry, durable, and porous material, is suitable.
A perfectly inelastic road bed is not to be desired. Something is necessary to absorb the shocks given by the wheels, and prevent their reaction against the machinery. To supply this amount of elasticity, and to transmit the weight evenly to the ground, is the duty of the ballast and timber-work.
Of late years there has been applied, in England, cast-iron hemispherical bowls, designed to take the place of both tie and chair. Such answers very well when there is no lack of ballast, and where wooden ties are worth from seventy-five cents to one dollar each.
SECTION OF THE RAIL.
286. A good rail must be able to act as a girder, or supporter, between the ties, as a lateral guide upon curves; and must possess a top surface of sufficient hardness and size to resist the rolling wear of the wheels.
| Lbs. per yard. | Tons per mile. (2,240 lbs.). | ||
|---|---|---|---|
| One square inch | of rail section weighs, | 9.9 | 15.72 |
| Two inches | of rail section weighs, | 19.8 | 31.42 |
| Three inches | of rail section weighs, | 29.7 | 47.14 |
| Four inches | of rail section weighs, | 39.6 | 62.84 |
| Five inches | of rail section weighs, | 49.5 | 78.56 |
| Six inches | of rail section weighs, | 59.4 | 94.28 |
| Seven inches | of rail section weighs, | 69.3 | 110.00 |
| Eight inches | of rail section weighs, | 79.2 | 124.50 |
| Nine inches | of rail section weighs, | 89.1 | 140.01 |
| Ten inches | of rail section weighs, | 99.0 | 155.57 |
| Single line of rails. | Double line of rails. |
Thus, at sixty dollars per ton, each square inch of section costs $943.20 per mile, or $94,320 per one hundred miles, whence the necessity of rolling the rail to the form which shall give the greatest strength with the least weight.