Not to mention fuel efficiency. I'd like to bet that a five car 80x on diesel consumes less fuel than a Voyager with only a small difference in performance. Once under the wires then you're laughing.
I also ran the numbers on track access charges which are different enough to cover the cost of a few units even without any other benefits.
On the track access charges, assuming that the 7 coach sets don't count as full length the cost would be 280.46ppm for 2 services, whist a 9 coach and a 5 coach set would be 253.12 for two services.
For 5 million miles (and XC is likely to be double maybe even triple that) that's an extra £2,233,000 in track access charges. To put that into context that's 10 coaches equivalent of lease charges which would mean that the business case for the mixed fleet would be easier to justify whilst still giving the same hourly capacity in the core.
Now, it's worth bearing in mind that for the 80x's they have a lower still track access charge if they are "full length".
If comparing 2 sets which are 7 coaches vs 1 set of each of 5 and 9 coaches (i.e. if we are comparing a universal 7 coach fleet vs a 50:50 split 5 and 9 coach fleet - so the same number of coaches) and at 5 million miles the track access costs for the mixed fleet would be £1.35 million less in costs (or about the same as the lease costs for 6 coaches).
However, as previously highlighted, the capacity uplift of a universal 7 coach 80x fleet would be significantly more than is needed (read would cost a lot more and so significantly reduces the probability of the business case staking up - or to pit it another way leaving us with the 20x's and no capacity uplift).
The below details the number of coaches needed and then the fleet capacities:
The thing is to deliver a universal 7 coach fleet would require more coaches, and so would be the more expensive option, the whole point of the mixed 5/9 coach fleet was that it was a similar sized fleet in terms of coaches:
312 - current fleet
342 - current fleet plus the 6 in storage
304 - mixed 5/9 class 800 similar fleet (12×9 and 40x5)
332 - upgraded mixed 5/9 class 800 similar fleet (19×9 and 33x5)
364 - universal 7 coach class 800 similar fleet
If lease costs are comparable per coach the in service fleet seat count could rise:
13,024 - current 22x fleet
14,331 - current 22x fleet + the spares
17,630 - original 5/9 coach option
19,528 - upgraded 5/9 coach option
That means that compared to the agreed fleet (i.e. the spares) the upgraded 5+9 fleet option provides a 36% uplift in capacity (+50% vs the original fleet) whist needing 9% fewer coaches and paying less in track access charges.
As such, anyone putting forward the universal 7 coach fleet option, would need to explain how that would meet a better business case than a mixed 5/9 coach fleet.
Obviously, the best case would be to order a mixed 5/9 coach fleet but with an option to order the extra (and probably enough to have a mixed 7/9 coach fleet) to create the universal 7 coach fleet if passenger growth is higher than 50% (i.e. I'm not suggesting that we block having a universal 7 coach fleet, by not having the option to deliver it if needed, just that it's going to be very hard to justify it as the starting point for the new fleet order).