Soon after the invention of punch cards and tabulators for the 1880 census, railroads adopted them to track freight cars and bill customers. By the early 1900s, the railroads operated one of the largest information-processing systems in the country. In 1920, more than 1,000 railroads ran about two million freight cars over some 400,000 miles of track. Thousands of clerks kept records in file cabinets and specialized account books to ensure that each car got to the right place and that everyone received the correct bill.