I doubt if the bandwidth would be available, I know they used to run comms over the rails but I'm not really that clued up on it. Many years ago I knew a guy working on comms for BR, I think the technology then was a LF or HF signal. I also remember seeing some of this old equipment available at a radio rally aroundb that time. (thats how the discussion started)
It was a long time ago so perhaps I am confused and they were for use on overhead electrics. Any for of communication down the rails will be a problem, you have to consider loosing connections at points, and corrosion if not on a continuous welded section of track.
Also remember that the signal levels would need to be very high if using LF/HF in the same manner as ADSL, because the railway lines are not twisted pair
With twisted pair the crossing of the wires cancels out any unbalanced signal. ie: interference picked up on the line by using the push-push effect. (I'm sure you could Google for an explanation)
BT's lines were designed for audio signals, getting good balance at HF frequencies (that ADSL uses) is not an easy task. The old Rediffusion HF cable TV systems used balanced twisted pairs desugned for the job, but they still had problems with balance and interference.
The use of digital technology these days goes some way to mask the inherent problem of using twisted pairs for broadband communications. Twisted pair in itself is not a bad thing, it has a higher velocity ratio than coax, and has less loss/km. The real problem is maintaining good balance at all the joints, the use of chock block type connections ruins the balance.
Whilst a network can be efficiently maintained for audio or low speed data (voice) using it for broadband is far different. ADSL technology masks the problems with error correction, so its very much like trying to make a silk purse out of a sows ear.
Using the railway lines would only offer one ADSL line shared between all the users on that track, even if it did work satisfactory.