During the Second Industrial Revolution, as factories were built around train depots,

Train depots and factories were often located in or near crowded city centers, where working-class people lived close to their jobs. As transportation improved and suburbs grew, wealthier people could move farther from the industrial pollution and congestion while still traveling into the city. So the best answer is that wealthier people moved out of the inner cities; the working poor generally did not leave cities behind.

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