Train

In the United Kingdom, the interchangeable terms set and unit are used to refer to a group of permanently or semi-permanently coupled vehicles, such as those of a multiple unit. While when referring to a train made up of a variety of vehicles, or of several sets/units, the term formation is used. (Although the UK public and media often forgo 'formation', for simply 'train'.) The word rake is also used for a group of coaches or wagons.
In the United Kingdom Section 83(1) of the Railways Act 1993 defines "train" as follows:
a) two or more items of rolling stock coupled together, at least one of which is a locomotive; or
b) a locomotive not coupled to any other rolling stock.