The branch of transportation engineering that deals with feasibility study, planning, design, construction, and operation of subway (underground railway) systems. In addition to providing rapid and comfortable service, subways consume less energy per passenger carried in comparison with other modes of transportation such as automobiles and buses. They have been adopted in many cities as a primary mode of transportation to reduce traffic congestion and air pollution.
Subways are designed for short trips with frequent stops, compared to above-ground, intercity railways. Many factors considered in the planning process of subway systems are quite similar to those for railway systems. Subway system planning starts with a corridor study, which includes a forecast of ridership and revenues, an estimation of construction and operational costs, and a projection of the potential benefits from land development. See Railroad engineering, Transportation engineering
All subway systems have three major types of structures: stations, tunnels, and depots. The most important task in planning a new subway system or a new subway line is to locate stations and depots and to determine the track alignment. Subway lines are normally located within the right-of-way of public roads and as far away as possible from private properties and sites of importance. Because stations and entrances are usually located in densely populated areas, land acquisition is often a major problem. One solution is to integrate entrances into nearby developments such as parks, department stores, and public buildings, which lessens the visual impact of the entrances and reduces their impediment to pedestrian flow.
Design of the permanent works includes structural and architectural elements and electrical and mechanical facilities. There are two types of structures: stations and tunnels. For stations, space optimization and passenger flow are important. The major elements in a typical station are rails, platform, staircases, and escalators. For handicapped passengers, provisions should be made for the movement of wheelchairs in elevators and at fare gates, and special tiles should be available to guide the blind to platforms.
In both stations and tunnels, ventilation is essential for the comfort of the passengers and for removing smoke during a fire. Sufficient staircases are required for passengers to escape from the station platform to a point of safety in case of a fire. The electrical and mechanical facilities include the rolling stock, signaling, communication, power supply, automated fare collection, and environmental control (air-conditioning) systems. Corrosion has caused problems to structures in some subways; therefore, corrosion-resistant coatings may be required. To minimize noise and vibration from running trains, floating slabs can be used under rails or building foundations in sections of routes crossing densely populated areas and in commercial districts where vibration and secondary airborne noise inside buildings are unacceptable. See Ventilation
Underground stations are normally constructed by using an open-cut method. For open cuts in soft ground, the sides of the pits are normally retained by wall members and braced using struts. The pits are fitted with decks for maintaining traffic at the surface. For new lines that pass under existing lines, it is not possible to have open cuts. In such cases, stations have to be constructed using mining methods (underground excavation). See Tunnel
Many modern subway systems are fully automated and require only a minimal staff. Train movements are monitored and regulated by computers in a control center. Therefore, engineering is limited to the function and maintenance of the electrical and mechanical facilities. The electrical and mechanical devices requiring constant care include the rolling stock, signaling, communication and broadcasting systems, power supply, elevators and escalators, automated fare collection, and environmental control systems. Also included are depot facilities, and station and tunnel service facilities. See Electric distribution systems, Railroad control systems