Electric locomotives use series wound DC motors (lots of torque at low speed...) on each driven wheel as the power source. DC motors will operate fine on AC, but as the frequency increases you run into commutation problems. As such, to gain the advantage of being able to transform voltages yet still operate the DC traction motors without intervening rectification, many railroads use low (lower than 50 or 60 Hz) frequency AC power.
I recall a tour of PECo's (now Excelon) Schuylkill Station in Philadelphia, probably in the early 70's. This was PECo's first central generation plant (it is still in service, mainly because the steam is used to heat center city and the University of Pennsylvania) and had a lot of old equipment retired in-place. Of particular interest was a pair of 60 Hz to 25 Hz conversion sets that were used to supply 25 Hz to the Broad Street Subway in Philadelphia. They had been retired only a few years before, but about 2 years after that there was a fire that destroyed the subway's converters, so they fired them up again for a couple of months until the subway system could be repaired. The really interesting part was that the stator on the 25 Hz side was mounted on a set of ring gears so that it could be rotated to properly phase it to the 25 Hz bus. The nameplate said it was a GE set, but when PECo tried to get the drawings for the archives, GE claimed they had no records of ever making the machines. Legend has it that Steinmetz designed it (they were old enough that it could have been true!).