Indeed. It is of course the reason most the HST workings to Worcester/Great Malvern were replaced by 166s but it isn't the reason for short term substitutions for HSTs.
Let's just get the sequence of events right, as you make it sound as though HSTs reigned supreme on the Cotswold Line for an extended period - it was more like a matter of months.
Until December 2004, soon after FGW took on the former Thames Trains routes, there were just the couple of Hereford-London and return peak HSTs, plus a few weekend HSTs doing the same journey, mostly on Sunday afternoons to take people back to London. Everything else was Turbo-worked
From December 2004, 180s took over most, but not all of the Turbo jobs, just as well given that peak and peak shoulder trains worked by Turbos were often full and standing on parts of their journeys, but growth in traffic meant that 180s soon filled up on some of these trains.
The 180s' reliability issues, plus the availability of HSTs freed from XC and West Coast led FGW to conclude they could get rid of 180s and use more 125s instead, so over the course of 2008-9 the 180s gradually disappeared, despite a degree of scepticism from the locals as to the wisdom of using 500-seat trains on many off-peak trains west of Oxford.
And so it proved, with the costs of carrying lots of empty seats around leading to the gradual removal in 2009-10 of HSTs outside the peaks, except the two Hereford services, and the return of 166s, as the 180s were no longer available to FGW - incidentally also affecting the Reading-Gatwick line, where the 166s had been moved, with their extra luggage space compared with 165s proving useful for the airport traffic.
I don't think senior FGW managers thought the 166s were what they should be using on Worcestershire services but there wasn't anything else more suitable available until East Coast ditched its Lincoln plan, freeing the five 180s that are now back with FGW.
When it comes to substitutions at Paddington, these are usually done at short notice, when an HST on another route has failed. And the crews on most of the Cotswold diagrams are trained on both HSTs and Turbos, so pinching one set off a doubled-up formation of Turbos is the obvious answer - you can't exactly send a 166 to Bristol or Cardiff. Four out of the five 180s are booked to be in traffic anyway, with the other on maintenance, so they can't help in such situations. A Turbo is never ideal but if that's all that's available we just have to put up with it.
SephenWorceste doesn't say when the journey he refers to was made - doesn't sound as though it was recent - so who knows whether it was to do with the flooding. During that period recently (and flooding at Oxford had more of an impact for the Cotswold Line than flooding elsewhere) and the signal problems in west London, the 180s pretty much vanished but with the scale of problems and crews being stepped up at a moment's notice, it made sense, as the crew training programme for 180s is still to be completed and a train without a driver is no use to anyone.
Turbo first class is not up to HST/180 standards but it was never intended for long-distance journeys. It has better seats than standard, made to a proper seat width. The 2+2 standard class section at tables in 166 centre cars just uses the same seats as the rest of standard class, so has a super-wide aisle and more legroom but that's about it.
Goatboy said:
Unless you were travelling on a First Advance and would rather have had the opportunity to catch the next HST rather than use Standard Plus on a Turbo, which you could do if it was cancelled but couldnt do if it was substituted
You just try finding an advance fare on the Cotswold Line...