radius + gaugeradius × length of rail.
The amount of this gain, or lead, is adjusted by cutting off a portion of the end of the inner rails at certain intervals.
Assuming the fish-bolt holes to be spaced as shown on [Fig. 342], then, when the inner rail is leading to the extent of 2 inches, a piece 4 inches long is cut off, as shown by dotted lines, leaving the original second fish-bolt hole to serve as first or end fish-bolt hole, and a new or second bolt-hole is drilled by hand at A. This method sets back the joint 2 inches from the square, and the lead is allowed to go on again until it becomes necessary to cut off another piece of 4 inches. Another mode is to have a proportion of the rails rolled 2 or 3 inches shorter for use on the curves.
On curves of a 1000 feet radius and upwards, the rails should be laid to the normal gauge, but on curves of lesser radius the gauge may be slightly increased, and as much as ¾ of an inch allowed on a curve of 500 feet radius.
The amount of cant, or super-elevation, to be given to the outer rail on curves must be regulated by the speed of the train and the gauge of the line. Many formulæ have been compiled to determine the necessary amount of super-elevation, but experience has shown that by some of them the calculated amounts were excessive. Possibly during past years too much cant has been given in many cases. The following simple formula approaches very closely to practical experience—
| (velocity in miles per hour)2 × gauge in feet radius in feet × 1·25 | = | the super-elevation of outer rail in inches. |
For high-speed trains uniformity of cant is of the utmost importance, more so even than the exact amount. Any irregularity in the super-elevation of the outer rail, sometimes high and sometimes low, will produce a dangerous swaying movement in the train, which, if not promptly checked, would lead to derailment.
More injury is done to curves by spreading, arising from rigid wheel-bases of engines and tenders, than from any want of counteraction to centrifugal force.
When a long length of permanent way has been linked in, rails spiked to gauge, and fish-plates bolted together, the platelayers can proceed to the final adjustment to line and level in accordance with the stakes and pegs provided for their guidance. The setting to exact line is effected by means of long pointed round iron crowbars, which are struck forcibly into the ballast alongside the rails, and serve as powerful hand-levers to pull or push the rails to the right or left as directed by the foreman standing some distance back at one of the line-stakes. The men with the crowbars pass from rail-length to rail-length, until a long stretch of road has been pulled into correct line.
The adjustment to rail-level is done by first packing up the sleepers to the correct height at the various level-pegs, and then packing up the intermediate sleepers so that the surface of the top of the rails forms one uniform even line from level-peg to level-peg. On new lines it is usual to pack a little high in the first instance to allow for the subsidence or compression which invariably takes place on the passage of heavy trains over fresh ballast.