It started here with both DC and AC distribution systems. AC eventually won because transformers would allow for transmitting the power for long distances at high voltages. Even for AC systems there were lots of different voltages and phasing. There are still small pockets of unusual voltages used with old equipment. Until recently one of our local power companies supplied 400VDC for elevators near a power plant.
NYC's (and nearby) subway systems and some other consumers run on 600 to 750VDC. Until not that many years ago, parts of the older (IRT) subway system signals ran on 25Hz AC. This is because electric railways were prevalent enough early on that there was a huge "installed base" problem which prevented conversion from DC to AC. In fact, the only system I know of that converted from DC to AC was the Newark City Subway, when they wholesale replaced their PCCs with LRVs.
This is an area I'm painfully familiar with. A transit museum I work with has its office and traction power from one electric utility (converted to DC from 3-phase 4160VAC with a rectifier set, although the old MG is still there and presumably operable). The tracks end there, but extend 1.5 miles in the opposite direction, in an area where all of the power comes from a different electric utility. So we have 600VDC overhead wire with return through the track on the ground. Some new buildings were put up down the line in that second utility's service area and electric service was ordered.
HUGE amounts of paranoia and paperwork all around. The electric utility wanted to run their 5KV power conduits at least 12 feet below the tracks. Unfortunately, that puts them below sea level. They wanted disconnects on both sides of the tracks. The planning department demanded disconnects near the building on both the input (utility) side of the transformer and the output side of the transformer, both before and after the metering toroids.
