RAILWAY CONSTRUCTION. 35 



gradients which at the best must necessarily be severe. In 

 some of the earlier mountain lines constructed abroad the 

 system of zigzags was introduced, as shown in * Fig. 25. 

 These zigzags were laid out on ruling gradients, one above 

 the other, on the sides of the mountain slopes with pieces of 

 level at the apices, A, B, and C, on which the engine could be 

 changed from one end of the train to the other. Although 

 feasible in principle, the system entailed considerable loss of 

 time in train-working, and was not unattended with risk. 



The more modern and simple method of working out the 

 same idea is to connect the main zigzag lines by curves or 

 spirals, thus rendering the route continuous and unbroken. 

 By this arrangement the heavy work and delay in starting or 

 stopping the train at the apices, A, B, and C, as shown on Fig. 

 25, is avoided, and the train can proceed continuously on its 

 circuitous journey. Fig. 26 shows an instance of the zigzags and 

 spirals, as carried out on an important railway abroad. To have 

 made a direct line from D to E, the most difficult part of the 

 route, would have involved a gradient of 1 in 11 ; but by con- 

 structing the spiral course, as shown, the length was more than 

 trebled, and the gradient reduced to 1 in 35. 



Fig. 27 is another example of spiral zigzags in which advan- 

 tage was taken to cut a short tunnel through a high narrow neck 

 of rock at G, and then by skirting round the hill the line was 

 taken over the top of the tunnel and along the side of the 

 mountain to the summit tunnel at H. By this means the line 

 from F to H was laid out to an average gradient of 1 in 42. 



Fig. 28 shows the Cumbres inclines on the Mexican Railway. 

 The route had to be located through one of the rugged passes of 

 the great Chain of the Andes, whose mountain-sides rise most 

 abruptly from the lower plains, to the great upper-land plateau, 

 some eight thousand feet above sea-level. The ground to be 

 traversed was so steep and difficult that, even with the best 

 available detours and greatest length that could be obtained, the 

 result was an average continuous gradient of 1 in 25 for 12 

 miles. 



Fig. 29 is a plan of part of the St. Gothard Railway, showing 

 the principal tunnel 9 miles long, and some of the adjoining 

 spiral tunnels. The long tunnel through the great Alpine 

 barrier was the only means of forming a railway connection 

 between the two points at Airolo and Goeschenen. Constructed 



