60 ~ 40Hz? Nope. We’ve been using 60 cycles since the beginning. Europe uses 50Hz. My iPad charger is rated for 50 & 60 and their corresponding voltages. Nothing else. You appliances and everything else would have serious issues with another cycle rate unless specifically designed for it.
"The Schoellkopf station complex eventually met with a very unusual fate. In 1956, it was almost completely destroyed by a massive rockslide from the towering cliff above. Following this catastrophe, 11 of the generators in Station No. 3A were rebuilt for 60-Hz operation, but Stations No. 3B and 3C were completely abandoned. "