The Budd Metroliner was a class of American electric multiple unit (EMU) railcar designed for first-class, high-speed service between New York City and Washington, D.C., on the Northeast Corridor. They were designed for operation up to 150 miles per hour (240 km/h): what would have been the first high speed rail service in the Western Hemisphere. Although 164 mph (264 km/h) was reached during test runs, track conditions and electrical issues limited top speeds to 120 mph (190 km/h) in revenue service. The single-ended units were designed to be arranged in two-car sets, which were in turn coupled into four to eight-car trains.

An electric multiple unit or EMU is a multiple-unit train consisting of self-propelled carriages using electricity as the motive power. An EMU requires no separate locomotive, as electric traction motors are incorporated within one or a number of the carriages. An EMU is usually formed of two or more semi-permanently coupled carriages, but electrically powered single-unit railcars are also generally classed as EMUs. The great majority of EMUs are passenger trains, but versions also exist for carrying mail.