Portion working

In rail terminology, portion working refers to the operational practice of coupling two or more passenger trains together over common sections of their respective routes, but otherwise operating the trains separately. This allows multiple trains to use the same path over parts of the network where limited capacity is available.
Typically, the common sections will be the busiest parts of the totality of the routes covered by a portion working arrangement.
History.
The practice of portion working has been followed for a long time on the third rail network of lines in the South East of England, and has been more widely practised in continental Europe.
Elsewhere in the United Kingdom, the practice has been less common, because of a general reluctance to design the necessary modern signalling systems, and because of legal constraints on competition between operators.
Possible combinations.
A variety of portion working combinations is possible. For example:
Issues.
For portion working to be successful, the operator may need to address many issues. For example: