 than communications. Network industries have historically been subject to more regulation than other sectors of the economy, including regulation by various transportation agencies, the Federal Communications Commission, the Federal Energy Regulatory Commission, and state public utility commissions. The components of transportation are air transportation, railroad transporta- tion, water transportation, truck transportation, transit and ground passenger transportation, pipeline transportation, other transportation and support activities, and warehousing and storage. Communications is composed of telephone and tele- graph and radio and television. Finally, the sector also includes water, sanitary services, natural gas, and electric power transmission. Not all of these elements represent network contributions, because communications includes production of content such as programming, and electricity services include the production of electric power. The Transportation sector is composed of network industries. Each mode of transportation is provided using networked facilities. Railroads are the classic The Structure and Functions of Networks example of a network industry. The nodes of a railroad include railway stations, railyards, and switching facilities, whereas the links of the system are the rails and the trains that travel on them. Railroads offer both passenger travel and freight transportation services. In 2007, railroads operated over 160,000 miles of track and had 1.77 trillion ton/miles of traffic (Association of American Railroads 2008). In the airline industry, the nodes of the network are airports and the links consist of the airline routes and the fleets of airplanes that travel those routes. In the trucking industry, the nodes are warehouses and pickup and delivery points, with links consisting of routes and the trucks that carry freight. The energy industries depend on large networks for transportation and trans- mission services. As of 2006, the U.S. natural gas pipeline network had nodes consisti