On the UK rail network, 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.
|Coupling code||System||Class of locomotives|
|Blue Star||Electro-pneumatic||Class 15, Class 17 (Nos D8588–8616), Class 20, Class 21 (Nos D6138-6157), Class 24, Class 25, Class 26, Class 27, Class 31/1, Class 33, Class 37, Class 40, Class 44, Class 45 and Class 46, Class 73 (under diesel power only), Class 74 (under diesel power only).|
|● Red Circle||Electro-magnetic||Class 16, Class 21 (Nos D6100-6137), Class 28, Class 29, Class 31/0.|
|● Green Circle||Some Class 47.|
|■ Orange Square||Diesel hydraulic - 1st use||Class 22 (D6300 - 6305), D600 Warship|
|Symbol re-used for different system||Class 50|
|Red Diamond||Class D16/1 (nos 10000/10001)|
Class 17 (D8500–D8587)
|Symbol re-used for different system||Class 56, Class 58|
|White Diamond||Diesel hydraulic||Class 22 (D6306 - 6357), Class 42, Class 43 (Warship Class)|
|Yellow Triangle||Diesel hydraulic||Class 35|
|SR EMU System||Class 33/1, Class 73, Class 74, Mark One Electric Multiple Units, Class 442, Class 489|
|AC electric locomotives||Class 87, some Class 86ss|
|Within own class only||Class 43 (HST), Class 60, some Class 68s|
|TDM System||Class 86, Class 87, Class 89, Class 90, Class 91, Push-Pull fitted Class 47, DBSO, DVT|
|AAR System ||Class 59, Class 66, Class 67, some Class 68, & Class 73/9, plus several converted DVTs|
First-generation diesel multiple units had the additional problem of differing types of transmission. For instance, a Class 127 unit (hydraulic transmission) could be required to work in multiple with a Class 112 unit (mechanical transmission). For this reason, the drive selector on the Class 127 was fitted with positions marked "D, 3, 2, 1" to change the gears when working in formation with vehicles with mechanical transmission.
First-generation DMU coupling codes:
|■ Blue Square||Most units with mechanical transmission|
|Orange Star||Most units with hydraulic transmission|
|Yellow Diamond||Derby Lightweight (mechanical transmission)|
|Red Triangle||Derby Lightweight (hydraulic transmission)|
|O White Circle||Class 126|
Most second-generation units built by British Rail were designed to use the BSI multiple working system, including members of the 14x Pacer and 15x Sprinter families. Some post-privatisation trains such as the Class 168, 170 and 172s were fitted with BSI couplers enabling them to operate in multiple with older stock, while other incompatible systems emerged. Examples included Dellner-couplers fitted to Class 171, 220, 221, 222, 350, 360, 375, 376, 377, 390, 700 and 710s while Scharfenbergs were fitted to Class 175 and 180s. Franchise changes and stock reallocation means that many train operating companies use fleets with a number of incompatible multiple working systems.
References and sources
- British Railways Diesel Traction Manual for Enginemen. British Transport Commission. 1962. p. 184.
- US Loco MU Control Archived 1 February 2008 at the Wayback Machine
- "Built in the 1960s… rebuilt for the 21st century". Rail. Retrieved 20 January 2020.
- The Railcar Association. "Description of the Class 127s". Retrieved 22 August 2007.
- Class 172 Angel Trains
- Class 390 Angel Trains
- Class 175 Angel Trains
- Class 180 Angel Trains