Railway overhead lines: What happens to other two phases in single phase railway power?
All phases of the three phase mains are used to create the single phase 16.7 Hz railway system. Back in the day there did exist separate power stations for railway but these days it’s all rotating converters (still in use, no new production) or inverters (think back to back HVDC station but only single phase output on one end and relatively low power). 

The Swedish Wikipedia article was excellent here. 


https://sv.m.wikipedia.org/wiki/Matning_av_kontaktledning
Perhaps try to translate it online?