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 192 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 193 yard, also a section of a 45-lb. steel flange-rail, much used on 3-foot narrow-gauge railways.