It’s a matter of practicality.
Departures off Long Rock in the morning:-
5A75 9-802, works 0505 London
5C53 4x 2-150, splits for the 0519 Truro (4) and the 0658 Ives (4)
5E12 4-HST, works 0600 Exeter
5A77 5-802, works 0541 London
5A79 9-802, works 0647 London
5S42 5-221, works 0635 Glasgow
5P60 4-HST, works 0730 Plymouth
5A81 5-802, works 0741 London
5S47 5-221, works 0837 Glasgow
5A82 5-802, works 0900 London
The 9 car for 5A75 is formed off the last arrival, blocks everything else in and cannot be serviced. It has to go back out on the first departure.
That leaves just one 9 car “available” for the remaining departures. Of those I’d say the 0647 is much busier through Cornwall than the 0900. There isn’t room on Long Rock for either a 3rd 9 car (there isn’t really room for 2), or an additional set to make the 0900 a 10 car - there just aren’t the roads on the depot, it’s not Old Oak Common.
So options - you could, theoretically, run a 5 (or a 9) down from Plymouth to attach to the 0900 - as happens with the 1000. But it would have to run down ahead of the sleeper, which would cut into engineering time, engineering time which becomes more critical as the service becomes more intense on the Cornish mainline. It also relies on there being a set available at Plymouth, with minimum contracted down periods on depots for the 80x fleet, I’m not sure there’s an early enough arrival onto Laira to get a set out to run down ahead of the sleeper for the 0900.
Or you could leave a set in the platform at Penzance for the 0541, making more room on the depot - but you’d have to get that set to Penzance somehow and you’d effectively lose a platform while it was stabled there.
At the end of the day the 0900 is busy through Cornwall, but let’s not over-egg its popularity. It isn’t leaving passengers behind as a 5 car. And if, as suggested above, efforts are concentrated on fixing the problem - namely issues coupling at Plymouth; then long term there shouldn’t be issues of it running 5v10 from Plymouth.
In an ideal world, if there were more 9s in the fleet; and if more 9s could be stabled down at Penzance then yes, more of the Cornish services would and should be 9s. But that isn’t where we are. The best has to be made of the fleet that exists. And while it may seem like Cornwall gets a raw deal with portion working compared to the North Cotswolds or Swansea - which equally don’t “deserve” all of the full-length trains they get - the difference is Plymouth station. Neither Oxford nor Cardiff are particularly conducive to portion working, with only 1 and 2 full length up platforms respectively for trains to attach in; and a much, much more intensive service off those platforms. The more relaxed pace of Cornish train services means a huge station like Plymouth really isn’t used to its full potential - which incidentally was designed to handle portion working of through trains to/from Cornwall.
Although the 800s have been in service for 18 months; portion working at Plymouth was much later to the party; the West of England services being the last to come over from HST operation. It happens day in, day out at Paddington with very few issues - anecdotally 2 or 3 a week from dozens upon dozens of coupling and uncoupling events there. The issues seem very much centred on Plymouth unfortunately. But it used to be awful at Swansea, with the daily couple/uncouple of the Carmarthen frequently failing. Hitachi and the local drivers team worked together to iron out what were train-borne issues; what was drivers’ technique and what was other factors - and now I can’t recall the last coupling/uncoupling failure at Swansea. It’ll get better with time; we just need to all be on the same page.
