subways

Subways are urban rapid transit systems that use dedicated rights-of-way for high-capacity trains, typically running underground for much of their route but also on elevated or surface sections. The term subway is common in North America, while many other regions use metro or underground to describe the same kind of system.

The modern subway emerged in the 19th century, beginning with London's Metropolitan Railway, which opened in 1863 and was initially steam-powered before electrification. The concept spread rapidly, leading to dense networks in major cities around the world in the 20th century. Today, subways operate in hundreds of cities across all continents.

Most lines are grade-separated from other traffic, which allows high frequency and rapid speeds. Trains are usually electric, drawing power from a third rail or overhead catenary, and are guided by fixed rails. Modern systems employ advanced signaling, such as automatic train control and communications-based train control, to coordinate service. Rolling stock typically consists of electric multiple units designed for frequent starts and stops and high passenger loads.

Subways play a central role in urban transportation, supporting dense development, reducing road congestion, and providing fast connections between business districts, residential areas, and major transit hubs. They can influence land use and foster transit-oriented development around stations. Operating hours vary by city, with many networks offering daily service and some providing extended or round-the-clock service on at least portions of the network.

Challenges include aging infrastructure, the high cost of construction and maintenance, safety concerns, and the need to upgrade signaling and accessibility. Despite these issues, subways remain a core element of public transit in many regions, continually evolving with new technologies and extensions.