Final answer:
Railroads and streetcars changed cities by allowing for the expansion of suburbs, decentralizing cities, and transforming transportation systems with the introduction of subway systems and elevated trains.
Step-by-step explanation:
Railroads and streetcars played a significant role in shaping cities. With the invention of the electric streetcar, cities were able to expand in size as people could live farther from their place of employment as long as they lived within walking distance of a streetcar line. This led to the development of streetcar suburbs and the decentralization of cities. Additionally, the introduction of subway systems and elevated trains further transformed the spatial layout of cities.