Track circuit calculation

[Murugesan india]

Dear Members,

I have a doubt regarding track circuit calculation.

Given : The minimum ballest resistance RB= 2.5 ohm.km

To calculate the length of track circuit L,

The formula is RB=(ρL)/a

where
ρ = Resistivity ohm meter
L = Length in meter
a = Cross sectional area in square meter

if we know the ballest resistance, how to find the length of track circuits.

i have verified in one of the calculations

the sample calculation conveys me

RB=ρ/L

how this is derived?
what abt the cross sectional area?

I got confused regarding this kindly clarifiy and also
what is the difference between ohm.km & ohm/km ?

Regards,

Murugesan