Early electric locomotives used "brush-type" AC/DC motors that operated much more efficiently at lower frequencies.
Small-signal models or HIL (Hardware-In-the-Loop) simulation results testing the stability of 15 kV / 16.7 Hz networks. Research Papers: Studies on the historical transition from low-frequency-16.7z
Hz to the modern 16.7 Hz standard to prevent synchronism issues in rotary converters. low-frequency-16.7z