242 TRACKWORK 



bent rails wr are called wing rails; the narrowest part mp of 

 the frog is called the throat. The throat of the frog must be 

 wide enough to allow the flanges of the wheels to pass through; 

 it is usually made about 2 in. wide. 



Frog Angle and Frog Number. The angle acb. Fig. 3. 

 between the outside edges of the tongue of the frog is called 

 the frog angle. This is also equal to the angle dee between the 

 outside edges of the tongue produced beyond c. The frog 

 angle which is represented by F is also equal to the angle 

 between the two tracks. 



The distance ab between the gauge lines at the end of the 

 tongue is called the heel width; the distance de, the mouth 

 width. If sch is the bisector of the angle F, the distance ch 

 is called the length of frog. 



The ratio of the length to the heel width is called the frog 

 number, and is usually denoted by n; that is, 

 n = ch -j- ab 



The relation between n and F is expressed by the formulas 



n-\ cot \F 

 and cot }F = 2n 



Frogs are usually designated by their numbers; thus, a 

 No. 8 frog is one in which n = 8. 



If the distance sh and the widths ab and de, Fig. 3, are meas- 

 ured on a frog, the frog number n can be determined by the 

 formula sh 



*~ ab+de 



Guard-Rails. Guard-rails, which are usually from 10 to 15 

 ft. long, are placed opposite the frog on the main track and the 

 switch track, as at R and R' in Figs. 1 and 2. The clear space 

 between the head of the guard-rail and the head of the main 

 or the switch rail should be about 2 in. 



