Let the actual distance travelled by train be 'd'. Similarly, actual speed and time be 's' and 't'

Given, If the train had been 6 km/hr faster, then it would have taken 4 hours less than the scheduled time,

st = (6 + s)(t - 4) (or) 6t - 4s -24 = 0................(1)

If the train were slower by 6 km/hr, then the train would have taken 6 hours more than the scheduled time,

st = (s - 6)(t + 6) (or) 6t - 6s + 36 = 0...............(2)

Subtract equation (1) and (2)

2s = 60 (or) s = 30

Substitute the value of 's' in equation (1)

6t - 4(30) = 24 (or) 6t = 144 (or) t = 24

The length of the journey (or) total distance = 24 x 30 = 720

Hence, option C is the correct answer.