What benefits might the railroad tracks give to the British Fourth Army?

I believe the answer is: Allowed for greater mobility; maining troops and faster resupply.

The railroad tracks provide a medium of transportation that can be used to move groups of army from one place to another in a short period of time. The railroads were also used as a method to send military weapons and food supply to the troops that need it in the battlefield.

Even if the British Fourth Army couldn't use the railroad tracks, it still provided a trail for them to follow to the largest cities.