There is also the benefit of increasing train frequency when pathing is difficult. I will take an example from the ECML. You can have 2x5 car leaving Kings Cross, which split at Doncaster. One goes to Leeds, the other to Newcastle. You get the extra capacity required on the crowded bit south of Doncaster, and increased frequency to Newcastle and Leeds, with the Kings Cross-Edinburgh being 9 car. I expect similar thinking would work well on the GWML, perhaps 2x5 car to Bristol Parkway, one set can then go to Wales and the other to the west country.