Wasn't there also a "pool" of spare power cars?
There were originally 95 complete HST rakes built, so it is true that there was a small number of spare power cars constructed.
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As I understand it, Western region Class 253s were lengthened all to 2+8 with new build in the 80s rather than displaced stock?
The introduction of the TGS vehicles retrospectively after all the HST rakes had been delivered had an impact on the formations of the trains I think in general that the TGS carriages replaced a trailer standard, although some of the first sets delivered had been formed with a pair of catering vehicles that was a very short lived arrangement.
I think that Ill probably do a bit of back reading later to refresh my memory of what precisely have been the main developments in terms of HST formations over the years, but in short as has been adeptly described by other posters above the HST sets are all longer now than they were when they were delivered: All of the HSTs except for Grand Centrals operate, or could operate, in 2+8 formation (Crosscountrys HSTs operate as 2+7 sets of course but have sufficient spare standard class vehicles to operate as 2+8 as was initially proposed when they were refurbished), with East Coasts sets further lengthened to 2+9 formation, when only the East Coast class 254 trains were originally delivered as 2+8 formations and all of the Great Western and Crosscountry sets were originally delivered as 2+7 sets.
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Ok, so the following has occurred to the production HST fleet:
There were originally 95 complete sets, delivered as follows:
27 sets (2+7) Great Western
32 sets (2+8) East Coast (Including 5 allocated to the Scottish region)
14 sets (2+7) West of England
18 sets (2+7) Crosscountry
4 sets (2+8) East Coast (Additional)
The individual vehicles were split into the following types:
Catering vehicles
37 x TRSBs Numbered in 400xx series, later renumbered in 404xx series.
20 x TRUK Numbered in 405xx series. Order cut from 27 carriages.
Quickly rebuilt as loco-hauled RFMs, Royal train vehicles and a couple of FOs.
57 x TRUB Numbered in 403xx series, reclassified as TRFB in 407xx series.
Leaving 94 catering vehicles after the TRUKs had been converted.
167 x FO
339 x TSOs
Later joined by 102 TGS vehicles
A total of 702 carriages when the TRUKs are discounted.
The HST sets are now formed as:
Great Western 53 x 8-car sets (424 carriages)
And 17 spare vehicles (441).
East Coast 14 x 9-car sets (126 carriages)
And 5 spare vehicles (131).
Midland Mainline 10 x 8-car sets (80 carriages)
And 4 spare vehicles (84).
Crosscountry 5 x 7-car sets (35 carriages)
And 5 spare carriages (40).
Grand Central 3 x 6-car sets (18 carriages)
And no spare carriages (18).
This gives a total of:
170 FOs.
365 TSOs.
74 full catering vehicles remaining, and 19 micro-buffets converted from TSOs employed by FGW.
92 TGS vehicles remaining in passenger stock.
And 5 TCKs converted for Crosscountry in the 45xxx series from loco-hauled mark 3 TSOs.
A total of 725 carriages.