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

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.

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.