Passage Eight（The Development of Cities）
Mass transportation revised the social and economic fabric of the American city in three fundamental ways. It catalyzed physical expansion, it sorted out people and land uses, and it accelerated the inherent instability of urban life. By opening vast areas of unoccupied land for residential expansion, the omnibuses, horse railways, commuter trains, and electric trolleys pulled settled regions outward two to four times more distant form city centers than they were in the premodern era. In 1830, for example, the borders of Boston lay scarcely two miles from the old business district； by the turn of the century the radius extended ten miles.