Matt BlazeRotary Converter, IRT Subway Substation 13, NYC, 2017.
High voltage pixels at https://www.flickr.com/photos/mattblaze/32992380451
NYC's IRT subway, opened in 1904, is powered by a 600 volt DC third rail running alongside the tracks. Power is fed to the system via a number of substations throughout the city, where high voltage AC is converted to the lower voltage DC used by trains.
Until recently, this was done with electromechanical rotary converters (essentially a combination AC motor and DC generator). They have been supplanted by solid state rectifiers, but a few of the original rotary converters remain operational.
Space Hobo@mattblaze For a while between the two technologies, there were some mercury vapour rectifiers, which are the most Mad Scientist-looking devices ever.
@spacehobo Yeah. I think the subway skipped over those. Mercury vapor rectifiers definitely have a mad scientist vibe.
Phil Stevens 
@mattblaze @spacehobo The 1.2 GW DC link between our main islands used to have mercury rectifiers at either end. Now the job is handled by thyristors.