I'd argue it's clear that any short to medium-term decarbonisation strategies will be using discontinuous electrification - past this, it will be a case-by-case argument for lines which will justify electrification. Those most likely to pass this test will be higher-speed lines (above 100mph most likely), those with freight traffic which is unlikely to be decarbonised any other way, lines with the most intensive services, and any upgrades to enabler battery operation. Many of these lines will be longer-term projects, but it's clear that BEMU vehicles offer the fastest path to decarbonisation for much of the UK. There are some particularly obvious candidates for this currently, notably GWR's Thames Valley services, certain North West commuter services (Buxton, Huddersfield, Southport, Rose Hill Marple, CLC), and Leeds commuter services (Harrogate line, Pontefract line, Sheffield).
As for where this leaves electrification projects? Past the Midland Main Line, I would argue Oxford and Bristol are two of the best targets for short-term electrification - both lines are almost ready to be electrified, and provide strategic routes for decarbonisation (including longer routes such as Cardiff-Portsmouth, and local services such as the Severn Beach Line which both could be operated by battery traction following these upgrades). After these various reports from the
Railway Industry Association and
Chartered Institute of Logistics and Transport highlight what they view as other key routes - many of these building off from the Midland Main Line and completing the Great Western. Key priorities for electrification highlighted by these reports are Reading to Basingstoke, Birmingham to Leicester and East West Rail - all obvious candidates to continue from existing schemes underway, or the Great Western which should ideally be resumed. On top of this, Reading to the Mendip Quarries, and the longer Birmingham to Felixstowe corridor are highlighted as core freight routes requiring electrification, which would allow electrification for passenger services on these routes.
Obviously some further works will be required to continue decarbonisation further - but the rapid development of battery technology continues to reduce the amount of works required (for passenger services at the very least) - as far as passenger-focused projects go, Miles Platting to Rochdale looks to be an obvious win, allowing for many of the remaining diesel Manchester commuter services to be feasibly battery-operated (Atherton line, Clitheroe and Blackburn).
In any case an integrated strategy to replace life-expired rolling stock with battery-electric trains should be aimed for - which luckily looks to line up with the areas with highest potential for electrification. For GWR the seemingly obvious move would be a fleet of 20 or so BEMU units to cover Thames Valley services, allowing for Turbos to be moved West and replace Class 150 units. It would also be logical to cover East West Rail with this order, assuming Oxford electrification works are completed. For Northern, electrification between Miles Platting and Rochdale and an order for suburban BEMU units to run as many services as feasible should allow retirement of Class 150 units, and start to make headway towards replacing Class 155/156 units as well. The next units coming up for replacement would be Class 158 and Turbo units, and raises big questions over Chilter. Here I would anticipate discontinous electrification between Marylebone and Wycombe, much like the Valley Lines to help reduce the cost of electrifying expensive structures. By this point all services out as far as Aylesbury and Oxford could be moved across to BEMU units, with newer Turbostars continuing services to Birmingham. I suspect at this point, further electrification around Exeter might start to look appealing, allowing for local services to be converted to BEMU, and even providing continuous electrification towards to Bristol and Westbury with the higher speeds on these routes might look appealing. Depending on how fast-charging technology develops, this could present a solution for the Cornish branch lines. All in all, this probably takes us through the next 15 years or so of stock replacement and retirement of Sprinters and Turbos (probably with some cascades and shuffling of Class 170 and Class 185 routes and fleets to replace Class 158 units up north, and probably a new order for battery/bi-mode/dual-voltage regional units for the West of England).
