It is often claimed that this would be uneconomic, due to the leasing costs, and yet the new trains will presumably cost more when they arrive.
Trains
have been extended from 2 + 7 up to 2 + 8, so
FGW▸ have indeed added stock, but consider too:
a) Each diagram probably has overcrowding for only 30 minutes in 24 hours on average
b) Adding an extra coach costs lots of ^ - will it generate that much more business, or simply let people spread out a bit during that 30 minutes? I note that money was paid to FGW to do the lengthening, which gives us a clue.
c) Adding a peak diagram with stock with a different top speed / acceleration profile (even if it's a longer train) will probably take up multiple paths.
The ultimate current logic is (in theory) to take all trains up to same-type or same-profile, maximum length services on the main and
slow relief lines. That's going to be 2 + 8
HSTs▸ and paired (10 car) Adelantes on the main, and 165 / 166 units on the relief. Outward of Airport Junction, the last train before each Heathrow service to make an extra stop or two before it gets to Reading, thus providing the Slough / Maidenhead / Twyford expresses. Grand theory, and yet I expect pretty much the way it's being done save for train length.
So where could the extra carriages come from, IF it were economically or politically mandated? Consider we're looking for a shortish term solution as electric trains will cause a rethink. You could cascade - use
LHCS▸ on Cardiff - Taunton as has been done before, use the saved 15x units on branches such as Windsor, Marlow and Henley, and on Oxford - Banbury and Reading - Newbury stoppers, freeing up 16x units to lengthen shorten trains, and to fill the two diagrams vacated by joining up the Adelantes into pairs.
I'm being an amateur, armchair designer here and I can hear cries of "yes, but" going up, some of which may be show stoppers. For example, a longer train takes longer to pass over speed restrictions so may be slower, and if we put 15x units on the branches, do we end up ending through trains? Paddington to Marlow - would change at Maidenhead and change at Bourne End too be acceptable? I have no answers, but perhaps I'm providing some clues as to the complexities that the experts face in what might at first look like a good solution.