Multiple working

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On the UK rail network not all locomotives and multiple units have the ability to work together.

In the early days of diesel locomotives, locomotives worked within their class (ie, two locomotives of the same class could work together but not with other classes). Since then, locomotives have been built to work with other locomotives in the same code or system.

Code or system	Class of locomotives
AAR System	Class 59, Class 66 and Class 67.
Blue Star Coupling Code	Class 20, Class 25, Class 31, Class 33 and Class 37.
Green Circle Coupling Code	Some class Class 47.
Orange Circle Coupling Code	Class 50.
Red Diamond Coupling Code	Class 56, Class 58.
SR System	Class 33/1, Class 73, Mark One Electric Multiple Units.
Within own class only	Class 43 InterCity 125, Class 60.