Context: DC commutating electric motors, if fitted with laminated pole pieces, become universal motors because they can also operate on AC; reversing the current in both stator and rotor does not reverse the motor. But the now-standard AC distribution frequencies of 50 and 60 Hz caused difficulties with inductive reactance and eddy current losses. Many railways chose low AC frequencies to overcome these problems. They must be converted from utility power by motor-generators or static inverters at the feeding substations or generated at dedicated traction powerstations.

Question: How can DC motor turn universal?
Answer: if fitted with laminated pole pieces

Question: What problems did AC distribution cause?
Answer: inductive reactance and eddy current losses

Question: How do railways try to solve the problem of inductive reactance of AC system?
Answer: low AC frequencies

Question: How can low frequencies be obtained?
Answer: converted from utility power

Question: Reversing the current in both stator and rotor also reverses what?
Answer: Unanswerable

Question: Many railways choose high AC frequencies to overcome what?
Answer: Unanswerable

Question: The now-standard DC frequencies are what?
Answer: Unanswerable

Question: The distribution frequencies of 60 and 70 Hz caused what?
Answer: Unanswerable

Question: Many railways choose low DC what?
Answer: Unanswerable