I 9 2 RAILWAY CONSTRUCTION. 



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. 



