For some reason, when AC was to be used (needed for transmission efficiency), 25Hz was required for electric motors back in the 1920s and 1930s. It was widely used in railroads. The electric trains from New Haven to Washington DC, west to Harrisburg, and on numerous branches serving commuters, was all 25 Hz, and much of it remains 25 Hz to this day.
Subways used 600 V DC, but purchased or generated 25Hz power and sent that out to the substations for rectification and voltage transformation.
Because of the above, for years certain commercial power stations continued to generate 25 Hz for their railway customers. I think now they finally are able to use solid-state converters instead of the big rotary machines. As mentioned, some railroads (but not all) switched to 60Hz.