When I first published this page I found it strange that power was distributed at such a low frequency; one obvious objection to 16.6 Hz distribution is that all the transformers would be three times the size of those for a conventional 50 Hz system. The answer is that due to the state of electric motor technology at the time, electric locomotives used serial wound commutator motors as traction motors. They were essentially DC motors running on AC. Without using electronics, it is difficult to control the speed of an AC induction motor. As Fehér Tamás, one of my correspondents, put it: "About 18Hz is the highest frequency where sturdy-built DC electric motors can be run safely with minor modifications. There will be occasional sparks and arcs at the commutators, but it is tolerable. Go over 20 Hz and you will have frequent circular-fire shorts on the current feed brushes, quickly destroying the motor."
I would like to thank all those who wrote in to point this out to me, and I can only offer my apologies for taking so long to update this page.