Hi,
the transformer or the cable is the problem because they both generate a magnetic field. (the cable is probably the big problem) The frequency of the railroad is 16.5Hz in Germany. What we hear here as a disturbance are the overtones and their distortions (approx 600hz-5000khz … it is a whir). These disturbances are omnidirectional and no material can stop them, at most bend a little. (except µ-Metal in many thick layers … very expensive) The only solution is distance (approx. 80-100meters) or a magnetic field canceling system. Your cable is basically like our railway line. You can check this with a guitaramp with battery … and then walk away from the generator/cable and hear what happens.
Good luck
Greetings Michael