Correct Answer - Option 1 : 25 cm
Sleeper density is denoted as L + m.
Where L=Length of one rail; m= any integer
Sleeper density signifies the number of sleepers per one rail length.
Optimum depth of ballast cushion = (S-W)/2
Where S = Spacing of sleepers
W=Bottom width of sleeper
Sleeper density = (M + 5) and bottom width = 22.22 cm
Length of one rail = 12.8 m
No of sleepers = 12.8 + 5 = 17.8
Spacing of sleepers = (12.8x100)/17.8 = 71.91 cm
Optimum depth of ballast cushion=(S-W)/2 = (71.91 - 22.22)/2 = 24.84 cm = 25 cm