When the new trains arrived they were seen as the magic bullet - they had something to be proud of then. They were modern, accessible and comfortable, they all had 3 coaches whereas they replaced some that had 2, they could do great things in a timetable and there were enough of them.
Had they not suffered the very silly decision to stick the Scotland services in their franchise, many of their problems might not have been.
The original proposal from First/Kelios was 56 x 3 car trains for North TPE, South TPE and Manchester Airport to Cumbria services, with the option to lengthen some trains to 4 cars. That would have worked. However, the SRA decided to reduce the number of new trains to 51 x 3 car and 9 x 2 car (cascaded) but to make TPE run Manchester Airport to Blackpool instead of Northern and later made TPE run Scottish services to allow for Voyagers to freed up to allow for the VHF timetable on the WCML.
On paper the original proposal from First/Kelios seemed to be good, noting that the 3 car 185s have less seats than 3 car 158s and the 158s allowed more flexible formations, so lengthening some 3 car 185s to 4 car 185s would have allowed more flexible formations and services which previously had 3 car 158s to not have had a capacity cut permanently. However, it wasn't to be.
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From the FTPE Wikipedia article:
Maybe it's fair to say that 6 car trains would be adequate on most services rather than 8... but with this kind of prediction available, why did nobody plan ahead?
Well if you take Pendolino carriage capacities for an 8 car version with a higher proportion of standard class seating you could have:
Carriage A (driving car): 46 standard seats
Carriage B: 62 standard seats
Carriage C: 48 standard seats (plus shop)
Carriage D: 62 standard seats
Carriage E: 76 standard seats
Carriage F: 76 standard seats
Carriage J: 37 First Class seats
Carriage K (driving car): 18 First Class seats
That would give you 370 standard class seats, compared to 308 on a 6 car 185, so ignoring the service enhancements last year 8 car 125mph trains was possibly a good prediction.
As for why no-one planned ahead it's similar to other franchises. Demand has changed a lot since 2004 and TPE like Northern and other franchises has needed to be re-let on revised terms for a while. However, the West Coast fiasco meant everything got pushed back and have pushed potential improvements back by 2-3 years.