This is actually two questions. As my imagination I have the German/Swiss/Austrian/Norwegian/Swedish railway system on mind (15kV, 16.7Hz). I think conceptually the problem is the same on other AC systems.
So in order to operate a overhead AC system one must produce one phase AC power. Usually generators will however generate 3 phases of electical output. But for such a system you need only one. And it is understatement that a generator or 2-phase system must be operated with symmetric load. The problem with this system is that the system uses 16.7 Hz frequency (which is not equal to the mains grid with 50 Hz)
So what happens to the other 2 phases that are not used?
Secondly: Is the railway net actually running phase symmetric in all it's separate sections? Or could it be that sections are driven with phase difference (so to say the other phases)?