The Inverness portion is shared with Edinburgh, and Fort William + Aberdeen with Glasgow. As norbitonflyer explained, this is because coaches with accessible cabins are required on all portions - hence 2 for Fort William + Aberdeen. This reduces the overall capacity on this portion, which gets sent to Glasgow rather than Edinburgh as demand is less for Glasgow than the Scottish capital.
The Glasgow/Fort William + Aberdeen sets are always at the front on WCML because when the northbound Lowlander splits at Carstairs the front portion (logically) goes forward to Glasgow, however the southbound Lowlander combines at Carstairs as follows:
- 1C11 from Edinburgh runs through Carstairs and stops in the down passing loop
- 1M11 from Glasgow arrives at platform 2
- 1C11 is propelled onto the rear of 1M11
- 92 from 1C11 is removed from the rear of 1M11 and usually stables overnight at Carstairs to work 1B26, the morning Edinburgh portion.
This is unchanged from the mk3 days.
The southbound Highlander is usually assembled at Edinburgh as follows:
- 1B16 from Aberdeen arrives in platform 2. Locos detach and run to Craigentinny
- 1M16 from Inverness arrives in platform 19. Locos detach and run to Craigentinny
- 1B01 from Fort William arrives in platform 1, then runs into Calton north tunnel before being propelled onto the east end of 1B16 in platform 2
- 1B16+1B01 propelled into east end of 1M16. The ex-Fort William portion is split with the lounge and seated/brake coaches drawn back into Calton north tunnel before being propelled into a bay platform (usually 5)
One downside of the change from the mark 3 to mark 5 shunting chess is that northbound the Fort William lounge and seated coach/brake can't be shunted onto the portion until the Inverness portion has departed, reducing the scope for recovery from late running. (Bear in mind that once these Fort William coaches are added the locos then need to run round, made even more complex when a 66 is in play - as it usually is - as the 73/9 needs to be the coaching stock side of the pair at all times)