If I understand it correctly, some of the difference in price is due to payment being per-diagram, rather than per-day and Agility trains takes a hit if they don't provide enough trains to cover the contracted diagrams or if a train isn't able to complete it's diagram due to a technical issue.
The storage charges are so high because Agility trains would have provided the units to cover the diagrams so the DfT will be paying for maintenance (and I suppose fuel) that isn't needed.
Absolutely - but my understanding is that fuel is billed separately as it wasn't possible to predict fuel prices reliably over the next 27.5 years. It's also a bit more nuanced than my simple 'back-of-the-envelope' calculation. For example, Virgin pays for maintenance and cleaning separately but, for the IEP order at least, these are included in the payment-per-diagram and so are included in the global figure. It's also not clear whether the NAO figure covers only the scheduled diagrams that couldn't be used or whether it covers the whole electric fleet.
Whatever, even allowing for all of this, it wouldn't halve the cost per coach per day. The IEP order is still expensive.
In addition, because of the way the DfT has passed essentially all the risks to Agility Trains, Agility has ordered more trains from Hitachi than would seem necessary using normal availability figures to ensure there are a couple of spares available at all times. Coupled with this it has been reported that because of the maintenance agreement between the DfT and Agility (the TARA?) concerning the period of time allowed for maintenance for each train they will be stopped
longer for maintenance than the HSTs. This in spite of having electric brakes which mean the brake pads won't need such frequent replacement and, at the time the agreement was made, 21 of the trains were electric so they wouldn't need fuelling (apart from the donkey engine). So that's another extra train or two.
It will be fascinating to see the percentage of the whole fleet scheduled to operate each day.