BETWEEN THE END of the Civil War and World War I, the massive development of America's railroad system brought about a necessity for complex health care delivery systems, including contract practice and group hospitalization, to treat workers and passengers. The Baltimore & Ohio Railroad may have used a physician's services as early as the 1830s. By the time of the Civil War, the Chicago & Galena Union, Illinois Central, Chicago & Milwaukee, Michigan Central, Michigan Southern, and Lehigh Valley railroads all employed physician-surgeons on a part-time basis. As rapid growth took the railroads farther into the undeveloped areas of the West, it became necessary to implement medical services overseen by full-time, "chief" physician-surgeons. The hiring of both chief and local physician-surgeons was motivated not only by the extremely hazardous conditions of railroad construction but also by companies' interest in protecting themselves from increasing numbers of lawsuits. Thus, by the 1890s, most of the major railroad carriers had or were in the process of developing some form of in-house medical organization. By World War I, nearly 14 000 physicians (approximately 10% of the nation's total supply) worked either full-time or part-time for railroad companies.