The Pennsylvania Railroad used 25Hz 11,000 Volt from Washington to NYC and are major branches, including commuter lines. The Reading Company's Philadelphia area commuter network uses a similar system. The pre-war locmotives and self-propelled cars required that type of power supply. To this day that power continues although all the equipment is modern.
Today's equipment uses rectifiers to convert the AC power to DC. Newer designs use a modern type of AC motor; I believe the frequency is varied by solid state control units to control speed.
There was some talk of converting this system to more modern power but that would be costly and not gain anything. The most modern equipment has switches and can run on various types of power, older equipment must be modified in the shops.
The Reading Company had its own rotary converters and now has solid state converters. Amtrak, which inherited the former PRR installation, may still get 25Hz from PECO and other suppliers or convert itself (I'm not sure of the latest arrangements).
The Erie-Lackawnna suburban NYC commuter service used DC. It was converted to 60Hz in a rebuild; the advtg is this is commercial power and can be taken right off the grid. The above installations have fewer input points and have their own substations and distribution lines.
The New Haven RR also used 25Hz 11kv but too was converted to 60 Hz. Amtrak extended the electrification to Boston.
I wonder how the telephone company rectified AC power for its DC batteries. For large installations, a motor-generator set known as a rotary converter was required. Early machines would need 25Hz as well. Later on mercury arc rectifiers came out.
Over the years the technology of power supplies dramatically improved. The size and cost of power converters had marked improvement.
I don't know if true, but someone told me that in Europe that didn't bother with three phase for house supply, but just give everyone 220 service. That does seem to be more efficient for house supplies.