That would be good, it could mean more 158s for Arriva and the Rosco responsible for the arriva 158s already has experience in installing ertms equipment in these units for the Cambrian.
These units will be somewhere around 45 to 50 years old by the time electric units can run to Exeter, additional electrification works elsewhere. The likelihood of them being cascaded, certainly for more than just the odd year here or there, is very low.
There's a possibility the scheme could be accelerated, probably starting at the end of CP6 (with the Basingstoke to Salisbury portion) and then following on into CP7-CP8, if there's a determination to remove the diesel stock from Waterloo on emissions grounds.
Your best bet for additional stock is if the Chiltern Main Line is to be the target of a larger GWML type scheme fitting in with the Electric Spine works during CP6-CP7, but there's no easy fixes when it comes to releasing more DMU stock quickly through electrification. The first new electrification schemes are likely during the CP6 (2019-2024) control period and for most areas, the new works will be towards the end of that period, not the start.
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All these proposals for electrifying left right & centre are great in principle, but in practice the last two governments have not planned for this increased need for non weather dependent power output. It's Government's fault but unless we restart coal or allow more gas fired stations we will not have the juice for these eminently sensible plans. Sensible bar Yeovil to Weymouth,where the passenger miles carried are mostly pitiful.
There's plenty of time - none of the discussed schemes are going to be drawing a kW of power out of the National Grid any time in the next decade, and most are two decades away.
They're being discussed for planning purposes, so we can get signalling works done, drainage installed, structures cleared, immunisation sorted, rolling stock plans in place and indeed, to work out the power demands and generation requirements.