The centre carriages are making the 168/1s 4 car again, and increasing the 2 carriage units up to 3 cars. The 165/166s could in theory be extended, but due to the age of the design, it's very unlikely indeed.
10 years? in a life of 25? Would have thought it was quite possible. I didnt know there were any 2-car 168's so it has to be the 165/0's whicha re increasing to 3-car sets. If thats true, they they are older than the 165/1's and 166/2's run by FGWLink.......
Thus no reason for being un-extendable.
Why would there need to be spare sets? FGWLink always a have 1 or 2 hanging around at Reading Depot due to work being undertaken on them.
There are indeed 2 car 168/2s - I think there's 4 of them.
The 165/166 units are no longer in production - the Turbostar units are based on them, but are not the same unit - they are compatible in multiple, but I'm not sure if a 168 intermediate car could run with a 165.
Why would FGW even think of lengthening them? As I see it with Thames Trains they worked far more services - everything at the moment + Oxford fasts, Cotswold line, Oxford - Bristols, Paddington - Sloughs. Now they have none of those extra services - Oxford fasts are are 180s & HSTs, Cotswold line are 180s, Oxford - Bristols are long gone, Paddington Sloughs have morphed into HConnect with 360s. New services? Well they're going half hourly to Gatwick - but with PSRs or their equivalents (meaning they cut stations instead of running more services) mean that those services shouldn't need many extra units - I guess 1 at most. The Turbos are now running in multiple much more frequently - most welcome - but I doubt the extra capacity that they might want in the peak could be justifed at most times of day, and so there's not much incentive to build anything new. Not to mention that FGW are unlikely to be spending money on much except their franchise commitments..