Niagara Mohawk ceased production of 25 cycle when the Schoellkopf Power Station, predecessor to the Niagara Power project, literally fell into the Niagara gorge. That was June 7,1956. Three - 70,000 horsepower and three - 32,500 horsepower generators, mostly 25 cycle were dumped into the Niagara River.
It was more cost-effective, due to the relatively low demand, to buy
25 Hz from Canadian Niagara than to build facilities to produce it.As of five years ago, Niagara Mohawk was still buying 25 Hz power from Canada and maintained a distribution grid in Buffalo. The NY Public Service Commission was allowing them to phase out the offering of 25 Hz. They were allowed to divide any costs associated with offering and maintaining 25 Hz service among the handful of remaining users. This was to be a disincentive to continued use of 25 cycle. They were also offering incentives for the users to retrofit their equipment to operate on 60 Hz.
I was working for a company that still had 25 Hz elevators and an ancient meter and panel. I was attending meetings with Niagara Mohawk to negotiate incentives to change to 60 Hz.
Again, this is 5 years ago. At the time there were a couple of scrap dealers, several businesses with 25 Hz elevators, a large frozen food facility, and a major producer of cereal products still using 25 Hz.