On the rail network in Great Britain, multiple working is where two or more traction units (locomotives, diesel multiple units or electric multiple units) are coupled together in such a way that they are all under the control of one driver (multiple-unit train control).

If the front locomotive of a pair in multiple has failed the driver can still control the rear locomotive for as long as air and electricity supplies are available on the failed locomotive.

Many main-line diesel-electric and hydraulic locomotives are capable of running in multiples of up to three under the control of one driver

— British Railways Diesel Traction Manual for Enginemen,

"In tandem" is when more than one diesel or electric locomotive are hauling a single train and under the control of a driver on each locomotive.

In the early days of diesel locomotives in the 1960s, locomotives worked within their class (i.e. two locomotives of the same class could work together but not with other classes). Locomotives from different manufacturers had varying methods of controlling engines or braking systems. If a train required more than one locomotive, an additional driver was needed, at extra expense.

Since then, locomotives have been built to work with other locomotives in the same code or system. Similar systems are assigned a coupling code, which is normally indicated on the front of the locomotive.

Early diesels were also fitted with communicating doors in the nose which allowed the secondman to access the train heating boiler of the rearmost locomotive. The doors actually saw little use and, as they frequently caused draughts in the cab, many of them were later welded shut.