Protecting circuit from railway's high voltage
I have monitored with a spectrum analyzer 200 meters from a traction train railway on CATV 300MHz cable TV. The ingress was huge and caused noticable interference when the train went by.  Most causes were earth ground oxidation to the coax and the quality of the coax ( single, double shield , moisture ingress from cuts) This quality is determined by a measurement of Transfer Impedance and CMRR or in other words ingress due to imbalanced differential lines ( signal and return) over the entire spectrum of interest and the induced Current on this differential inbalanced impedance.

The solutions are many. Low differential balanced impedance. High CM impedance greater than 5x the signal bandwidth or whatever does not cause group delay distortion.

To assume your twisted pair is only susceptible to transient voltages and not signal integrity is being naive.  If you solve signal integrity issues with a test using equivalent A/m impulse noise with xx ns rise time and V/m impulse noise with V/m and same rise time for coupling capitance, you can do near field experiments along your chosen signal cable and measure on a spectrum analyzer or DSO the induced signal into match impedances on your cable and extrapolate. 

Once you fully understand the ingress from these sources and can expertly simulate it with dV/dt and dI/dt levels from reactive switch tests then you can safely be protected from transient levels. Assume your ground connections age by 10 ohm to >100 Ohms over time unless you have a spec otherwise. better than powerlines and simulate this in your design and design verification test (DVT) plans.

Common solutions are STP cables, controlled earth ground impedance ( braid ,  flat wire or Litz wire ) CM chokes around cables at both ends and optimal impedance matched SMD Ethernet Baluns on board for each jack.

In my "green weanie" days as a junior design Engineer in Aerospace in late '70's,  I once made the faulty assumption that my "long" 9600 baud RS485 differential cable link was good at both ends for a rocket pre-launch SCADA network.  After I left the company and never had a chance to do a field trial, one engineer told me when ever the VHF transmitter was used, it caused all kinds of data errors.  Moral of my truth, learn from my mistakes and include a Pi filter at both ends to suppress demodulated  AM VHF/UHF from causing signal integrity errors which is basically a Balun with an RF load cap.  ( common mode choke)