Wiktionary
n. (context rail transport English) A train consisting of one or more self-propelled cars capable of couple with other units of the same or similar type and still being controlled from one cab.
Wikipedia
Multiple units (MU) are self-propelled train carriages capable of coupling with other units of the same or similar type and still being controlled from one driving cab. Often these are passenger trainsets consisting of more than one carriage. Single self-propelling carriages (also called railcars, rail motor coaches or railbuses) are multiple units if capable of operating with other units.
Multiple units are classified by their power source and are of two main types: electric multiple unit (EMU) or diesel multiple unit (DMU). Diesel-powered units may be further classified by their transmission type: diesel-electric (DEMU), diesel-mechanical (DMMU) or diesel-hydraulic (DHMU).
Locomotives utilising multiple-unit train control are not multiple units.
Usage examples of "multiple unit".
The multiple unit was as long as two city blocks and as high as a tall building.