Infrastructure
Most rapid transit trains are electric multiple units with lengths from three to beyond ten cars. Power is commonly delivered by a third rail or by overhead wires. The whole London Underground network uses fourth rail and others use the linear motor for propulsion. Most run on conventional steel railway tracks, although some use rubber tires such as the Montreal Metro and Mexico City Metro, and some lines in Paris Métro. Rubber tires allow steeper gradients and a softer ride, but have higher maintenance costs and are less energy efficient. They also lose friction when weather conditions are wet or icy, preventing above ground use of the Montréal Metro but not rubber-tired systems in other cities. Crew sizes have decreased throughout history with some modern systems now running completely unstaffed trains. Other trains continue to have drivers, even if their only role in normal operation is to open and close the doors of the trains at stations.
Read more about this topic: Rapid Transit