They'll go in the bin.
No need to cascade them to SN or SE. Its just not economically viable to add 25 year old trains to either SN or SE's fleet.
Agree. Once the 701 roll out is complete, there will be no need (or stabling capacity) for the 458's, in any guise on the SW network.
This has been done to death so many times, it is becoming boring. The 701 fleet (in numbers of cars) was designed to replace the entire SWT suburban fleet (ie; 455/456/458/707). The physical capacity of all depots and stabling locations on the SW network, was/is such that it was at its maximum at the end of the SWT franchise (2017).
All stabling locations in the suburban and outer* suburban areas (* inclusive of Woking, Guildford and Basingstoke) that had potential for extra siding space to accommodate 10 car formations, was all but exhausted with much expense going in to new/revised siding arrangements put in place at a number of locations.
The end game scenario is thus: everything on the suburban network goes over to 701 operation. Car (quantity) wise amounts to a very small increase over the 455/456/458/707 plan that once existed in reality. However, given that a far larger percentage of trains will be of fixed 10 car formation (701), this requires a substantial increase in 10 car capacity stabling sidings, ie, sidings that can accommodate a fixed 10 car train, without blocking or prohibiting the use of any adjacent sidings.
From a timetable perspective, trains must be resourced (stock and crew wise) from locations(s)/depots that are nearest and relevant to the service group in question. If those locations do not possess the required number of sidings that can accommodate 10 car fixed formations to provide said service........you tell me?
My point being that with all suburban services operated by 701 which will stretch stabling capacity issues even with Feltham in the mix, and all main line (elec) services Desiro (444/450) operated, I doubt there will be any space left to stable any other electric fleet type!
The likes of Clapham Yard and Bournemouth Depot have never been bottom less pits, and it has only been with due diligence of some within both planning and fleets departments in years past that total gridlock/timetable meltdown has not occured.
Apologies if this has come across as a slight rant.