Final answer:
Trolley cars in San Francisco were originally pulled by a cable system, introduced in 1873 to help navigate the city's steep hills and facilitate the development of middle-class communities.
Step-by-step explanation:
The original method that pulled the trolley cars in San Francisco was the cable system. This ingenious system, adopted from pulley technology used in the mining industry, was introduced in 1873. The cable cars were designed to navigate the city's steep hills and helped to transform the geography into elegant middle-class communities. This mechanism of transportation became a critical component in the development of the city and distinguished it from other forms of urban transportation that were evolving at the same time, such as electrified streetcars and elevated trains.