I already said that I don't want to drag this off thread so please forgive me. There is no train that could be built for this service that would make everyone happy within the current constraints because too many want/need to commute this distance and the number of commuters is growing.
We can either spend a fortune on the infrastructure, be it loading gauge, platform length, gaps between junctions and signals etc. or we will have to change the fact that so many people have to work in London and if they want a nice semi they have to live a long way away (With complicated travel arrangements).
I really think I have made myself pretty clear.
It is partially a British disease because in many other countries in Europe people don't seem to judge themselves as a failure if they don't have a mortgage on a house with a garden. They seem to cope far better with rented apartments.
The obsession with houses being an investment that you live in rather than a place to sleep in is not the whole problem. Most very large cities in the world have problems with commuting. The problem here is that rental of even small apartments near the major employment areas is a no no for anybody of average means, because dwellings for rent are part of another profit-driven culture. Municipal dwellings are becoming as rare as hens teeth.
So, back on topic, given that the above is the way it is and won't change much for decades at least, transport systems have to deal with the resulting tidal flows. Back to the microcosm that is the Portsmouth direct line crush:
1) the current demand effectively outstrips the capacity of the infrastructure to provide sufficient seats of any kind for all passengers (say) boarding inward of (or maybe including at) Guildford.
2) the demand is expected to increase over the next couple of decades or longer so the last seats may be filled at Haslemere, Petersfield or eventually even Havant/Portsmouth.
3) there is no political will (even from the Tories) to divert public funds at the levels required for increasing the basic infrastructure capacity.
4) for similar reasons, it is unlikely that the roads on the route will get much from public funds either.
5) the current rolling stock is either inefficient in seating capacity per path (class 444s) of maxed-out for seating capacity (class 450).
6) to increase the line's capacity, the only practical progressive steps can be:
a) increase all peak time trains to maximum length allowed for with current infrastructure, using SDO where necessary
b) replace the 444s with 450s (or similar) in high density seating configuration
c) to prevent passengers being unable to board, replace the 450s with stock that provides greater overall capacity but at the expense of some seating, (think a class 700 style of train).
7) A benefit of the progressive approach in 6) above is that rolling stock is at least re-deployable, so is not a dead loss when replaced. There are plenty of other lines in the UK that would be only too pleased to receive the Portsmouth direct line 'cast-offs' (especially if they were made OLE compatible) so they wouldn't be wasted.