Most trolley and subway systems used 600 VDC. Yet the power grid and most stand-alone power plants use turbo-alternators to generate AC power. One way to convert AC to DC is to physically bolt an alternating current motor to a DC generator. Obviously this is inefficient, and DC generators are brushed-motors, and the brushes would wear out and regularly have to be replaced.

By the 1930s, these rotary converters were replaced with mercury vapor arc rectifiers, which where large glass vats of mercury, that allowed current to only flow one way. In many ways they are much cooler looking as they glow -- almost something from a sci-fi movie. Being solid state without moving parts, mercury vapor arc rectifiers often saw service for many decades, well into the 2000s. See here: https://www.youtube.com/watch?v=QY6V2syGnZAI