Grids having to be synchronized is a good point, but could it be that back in the 1950s there were still some small areas which were not connected into statewide and national grids but just had a few power stations running a local network?
In England at this time, there were some cities in which the older, downtown districts were still running on their original DC distribution systems fed from a local power plant, even though outer, newer areas had been interconnected to the grid to get AC power.
Some West Virginia coal mines used power around 25 Hz as late as 1952 when my grandfather was a mine electrician.
That sounds very much like a local power plant along the lines I'm thinking about. Didn't many of the small mining places in WV start out as company towns? It seems quite probable in such a case that the mining company might have installed local generators to their own standards for the whole town.
Pensylvania Railroad used 11000VAC @ 25Hz for electric traction
25Hz was also used on some railroad lines in Britain in the past. The 25kV AC power supplied for all the main electrified lines these days is 50Hz though, with substations providing power from the main national grid.