Direct current, which is what all grids ran until about 1886 and most grids ran well into the early years of the twentieth century, uses an inflexible voltage set at the dynamo. If a grid is using direct current, then a streetcar simply cannot be run using the same generator or set of wires as a string of lightbulbs. Either all the bulbs explode (overvoltage) or the streetcar doesn’t budge (undervoltage); there is no happy medium.
