Just to summarise, as I can't see that it's been done in this thread elsewhere, the revised proposed order of IEP trains as of June 2012, as has been laid out in the Great Western and East Coast franchise consultations, has shown the following weekday unit requirement for IEP trains(The information contained in the consultations has been worded to suggest that the figures shown are for the number of units required in service on a weekday, rather than the full fleet size):
Great Western
Electric 8-car: 11 diagrams
Bi-mode 8-car: 12 diagrams
Bi-mode 5-car: 26 diagrams
Total: 49 diagrams
Delivery of the new trains and entry into service is expected to take place starting in early 2017, with the full fleet ready for service by early 2018. The first timetable period that will be able to exploit the performance of the new trains will be May 2018.
East Coast
Bi-mode 9-car: 10 diagrams
Electric 5-car: 10 diagrams
Bi-mode 5-car: 8 diagrams
Total: 28 diagrams
Introduction is expected from 2018, with the full fleet deployed from the start of the May 2019 timetable change.
This gives a total fleet size of:
11 Electric full-length sets (8-car) = 88 carriages
22 Bi-mode full length sets (8 and 9-car) = 186 carriages
10 Electric half-length sets (5-car) = 50 carriages
34 Bi-mode half-length sets (5-car) = 170 carriages
TOTAL = 494 carriages
Plus presumably some sort of maintenance contingent, so perhaps there might be a total of 30 Bi-mode 5-car sets on Great Western and 10 with East Coast.
It is expected that a 27.5 year contract will be signed with the Train Service Provider (Agility Trains) to provide maintenance facilities and servicing arrangements for the IEP fleets.