226 



RAILROAD LOCATION 



The grade elevation for Sta. 18 will be 155.5+.5= 156 ft. By 

 the same process, the grade elevation is found for each station 

 shown in the plat; and by means of interpolation between two 

 contour curves, points having the required elevation are 

 located opposite the corresponding stations of the preliminary 

 survey. Each point is marked by a small dot enclosed in a 

 circle. A line joining the points thus designated will be the 

 grade contour, or the line where the required gradient meets the 

 surface of the ground. The tangents AB and CD are then 

 projected so as to conform as closely as practicable to the grade 

 contour, and a suitable curve is inserted for the intersection 

 angle EFD. This is most conveniently done by means of a 

 curved protractor, an illustration of which is shown in Fig. 2. 



FIG. 2 



This instrument, which is made of transparent material, is 

 shifted until there is found a curve that will fit the topography 

 and will close the angle between the tangents, as required. 

 Curvature. There is no fixed rule for limiting curvature, 

 but for a permanent track it is desirable to have the curvature 

 as easy as possible. For all ordinary traffic conditions, it is 

 good practice to use such curves as will best conform to exist- 

 ing topographical conditions. Any curve up to 10 will be no 

 obstacle to a speed of 35 mi. per hr., the average speed of pas- 

 senger trains. This affords a range in curvature that will meet 

 the requirements of most localities. 



