Multiple Unit

A multiple unit is a self-propelled train or trainset made of two or more carriages joined together, in which more than one carriage carries its own means of propulsion, rather than power being concentrated in a single separate locomotive as in a locomotive-hauled train. The class covers both electric multiple units, which draw power from a third rail or overhead wire, and diesel multiple units, which carry their own onboard diesel engines and generators; the general multiple-unit configuration itself, rather than either power source specifically, is what lets one driver control the whole train through multiple-unit train control. Multiple-unit control dates to the 1890s: the Liverpool Overhead Railway introduced two-car electric units in 1893, and Frank Sprague developed the practical control system that was first applied to Chicago's South Side Elevated Railroad in 1897. Because of their strong acceleration, reduced maintenance complexity relative to a hauling locomotive, and quick turnaround from cabs at both ends, multiple units now dominate rapid-transit and commuter rail service worldwide, especially in densely populated areas requiring frequent stops.