Engineering colleges were established to feed the enormous demand for expertise. Railroads invented modern management, with clear chains of command, statistical reporting, and complex bureaucratic systems. They systematized the roles of middle managers and set up explicit career tracks. They hired young men ages 18–21 and promoted them internally until a man reached the status of locomotive engineer, conductor, or station agent at age 40 or so. Career tracks were invented for skilled blue-collar jobs and for white-collar managers, starting in railroads and expanding into finance, manufacturing, and trade. Together with rapid growth of small business, a new middle class was rapidly growing, especially in northern cities.