It's a mess thanks to the re-tractioning of the Class 455 fleet and the failure to prepare the ex Class 460 vehicles used to re-form into Class 458 units, or indeed, 15 years ago, the failure to actually reject the Class 458 units and procure an equivalent number of Desiro units.
It doesn't matter what way you do dual-voltage, either GWR is going to have to switch en-route to accommodate the DC sections, or SWT is going to have to switch en-route to use the route at all, assuming complete 25kV AC OLE provision.
If we're going to get on with DC to AC conversion, we're going to have to accept there will be lots of DC to AC changeovers happening with dual voltage units, so I see no good reason why we shouldn't use AC on the NDL and use dual-voltage units.
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If it goes ahead (AC) could a good chunk of SWT's Reading-Waterloo route get converted to AC either as part of the project or as a follow-on project?
Yes, but it will be rolling stock strategy dependent. Class 458s will be so close to being life expired it'll need to fit in with their eventual replacement. The oldest units will be 21 years at the start of CP6, and there's very unlikely to be any movement to wire Reading to Waterloo in either CP6 or CP7, which takes you to CP8 at the earliest, and the oldest unit is 31 years old.
If you then add in 2 to 3 years for electrification and testing works, the oldest units will be almost 35 years old by the time they head off to Alstom to be have 25kV AC capability added.
The more immediate priority is likely to be the push to Exeter from Basingstoke and then back to Farnborough before the Class 158 and Class 159 fleets are life expired (and as a push to remove diesel stock from central London for emissions reasons). The oldest of those are 27 years old now and would ideally be replaced in the next 10 years, though pushing through to 40 years is likely.