The "L" train first opened in 1892, when the South Side Rapid Transit line was inaugurated. At first, this line was driven by steam locomotives before being electrified in 1898.
The "L" train is powered by a 600 volt direct current (DC) third rail. Until recently this current was used to drive direct current motors in the "L" train's rolling stock. However, the "L" train is now being transitioned from DC to alternating current (AC), with the most recent line of cars (the 5000 series introduced in 2009) making this conversion on board. AC current is more efficient and allows trains to return energy created during braking procedures to the grid. Eventually, the whole system will run on AC power although the transition will take a number of years.
The "L" train is controlled by a combination of two-way signaling from the driver's cab of each individual train, automated towers and junctions and control centers. The control system is connected by fiber-optic cables and governs safety, scheduling and logistical issues.
