The NYC subway system plus 3rd rail operations into Grand Central Terminal are 600VDC. Not sure about other cities like Boston but wouldn't surprise me if they were DC too.
Most streetcars, light rail and underground is low voltage[0] DC, in Europe too. Until not too long ago, it was implemented with rotary converters.
In Germany, Switzerland, Austria, Norway and Sweden (and possibly a few more, but those I'm certain of) catenary voltage for normal trains is 15KV 16 2/3 Hz single phase. (Three-phase was experimented with, and a test track was built in Germany, where an AEG electric engine set a world speed record that took some time to beat.) It was chosen for its good match with the rpms convenient with the large motors of the time, and being 1/3 of 50Hz. For some railways in Sweden, like the "Malmbanan" (="ore railway") Luleå-Kiruna-Narvik, a special power station was built (in Porjus for the Malmbanan) that did generate the correct frequency in a single-phase system, but for most of the remainder of the country, there were rotary conversion systems fed from the 50Hz 3ph grid, the last such system delivered in 1978. They frequently were built moveable on railway cars, which normally were parked in bunkers adjacent to the line. (We sure like to put things we care about underground, we do.)
In the 80's, high voltage/power semiconductors for power modulation became good enough to permit conversion from 50Hz to 16 2/3Hz to be done in silicon, and that's been the norm ever since.
- In Sweden, "low voltage" is defined to be between 50 and 1000 volts. High voltage is above, and SELV is below.
A catenary designed today typically is at normal line frequency, and slightly higher in voltage, because that would be cheaper in the supply side. The engines make DC of it as soon as it leaves the HV transformer anyway, to make 3-phase variable frequency for the drive motors.
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