Up to the years 1870-1875, it was the common practice to make the top, or wearing surface of the rail, comparatively
round, as shown on the typical sections, [Figs. 263 and 267]. The effect of this sharp-curved outline was to limit the first wearing, or contact surface to a narrow strip along the head of rail, causing a tendency to groove or form hollows in the treads of the wheel-tyres. As the rail wore down, the upper surface assumed a much flatter curve, more closely assimilated to the section of the wheel-tyre, and giving better results for regular wear under heavy traffic. Profiting by this experience, the rails of the present day are made much flatter on the head than they were formerly, as will be noted from the sections shown on [Figs. 261, 266, and 269], which represent types of rails now actually in use on some of the principal railways.
In designing a rail for any given line, the section and weight of the rail must necessarily be influenced by the weight of the rolling-stock passing over it, and the amount of the traffic it has to sustain.
The engine, being the heaviest vehicle in the train, will give the measure of the greatest weight on one pair of wheels. Engines vary considerably on different lines, ranging from ten tons to eighteen tons or more on one pair of driving-wheels, according to the description of work to be performed.
Very often secondary or branch lines, with comparatively light traffic, have steep gradients, necessitating engines as heavy as on a main trunk line; but the number of trains on the former may not exceed twenty per day, while on the latter they may amount to one hundred and fifty or two hundred. It is evident that the rail which would last for very many years under the small traffic, would have a very short life under the frequent traffic. Hence the reason why it is found expedient to give a large increase of material in the heads of rails carrying the heavy, constant train service of many of our main lines.
[Figs. 261, 262, and 263] are sections of rails in use on lines having heavy engines and fast trains, but with a comparatively small daily train service, and [Figs. 264, 266, 267, and 268] are sections of rails carrying the heavy, fast, and incessant traffic of some of our leading lines.
On lines having small traffic, slow speeds, easy gradients, and comparatively light engines, a reduced section of rail may be adopted; but in doing so it is well to allow for any probable future development of traffic which might cause the introduction of heavier engines.
[Figs. 269 to 272] show sections of rails varying from 72 to 60 lbs. per yard, also a section of a 45-lb. steel flange-rail, much used on 3-foot narrow-gauge railways.
Valuable and interesting statistics have from time to time been recorded, with a view to ascertain the average life of a steel rail, by obtaining the number of million tons of train load which it would sustain before it became worn down to such an extent as to be no longer of service on the line. It will be readily understood that the rate of wear of a steel rail will depend not only on the weight and section of the rail itself, but on the class of rolling-stock, and the description of traffic it has to carry. It will also be largely affected by the circumstances of whether the line is on a level or on an incline.
The writer has had careful measurement taken of the wear of the steel flange-rail ([Fig. 265]), 79 lbs. per yard, and the result shows that with a traffic not exceeding twenty-four goods and passenger trains per day, one-tenth of an inch was worn off the top of the rail in ten years on the comparatively level portions of the line; but that the same amount of one-tenth of an inch was worn off in six years by the same traffic, on the same district of the line, in places where the gradients varied from 1 in 100 to 1 in 70. The heavy pounding of the engines, and the working of the brakes tend very materially to shorten the life of the rails on the inclines.