Ah but my proposed de-electrification would avoid even that expenditure, only requiring the labour and logistics to recover and dispose of the existing equipment, and the scrap rail, transformers and cables would probably have a value.
How about the labour and logistics associated with these new diesel engines. Assuming you can even fit them into the tube stock used now.
Otherwise you will need new stock which will cost far more.
You are looking about £1-2m for a new rectifier station capable of supporting the entire line.
This will allow for electric traction to continue as the cost of the conductor rails and such are minimal in this case as all the equipment is already in place.
Good luck getting your bespoke diesel gear for less than that.
Alongside all the expanded fueling facilities and maintenance support facilities to support diesel operation.
A large amount of new conductor rail!
Not particularly large - there is only about 14km of track on the entire route. And points at the extremities would not require new conductor rail as the current flows through them will be limited to only a single set for relatively brief amounts of time.
The value of aluminium is higher than steel but still so low [something like £1000/t] that £100k would easily cover it. Especially as it can use scrap conductor rail with the stainless steel almost worn through.
The use is so light that rail that would last months more on the main line would last years if not longer.
And if 25kV conversion of main line routes ever happen then this would produce huge quantities of scrap value rail with large quantities of remaining life that could be installed in the case of normal renewals.
We still have years before the electrification gear needs replacing to get it all in place after all.
With a dedicated track gang the replacements can fit in any empty slots in their jobs list.
A single centrally placed substation would have a maximum loop resistance to the end of line of only
180 milli-ohms (7km distant) using.
A feeder busbar running alongside the track in parallel with the running rails would cut this further but I won't include that - although it would cost very little and cut resistance to something approaching 100 milli-ohms.
The maximum length of train on the route is 6-cars. Which will have a formation peak power requirement of 1100kW.
At minimum permitted rail voltage of 450V that would mean a peak current of 2440A.
2440A across a 180 milli ohm circuit is a voltage drop of 440V.
Which sounds like an awful lot but 890V at substation is actually still inside the normal 3rd spec.
If we have that busbar in parallel with the running rails (you can make it out of scrap aluminium conductor rails if you want which as I said are available in significant quantities) then the voltage drop drops to about 220V, putting the substation voltage at 670V.
If you set substation voltage to 750V nominal then you end up with a train voltage of 550V, a current of 2000A and a far more reasonable loss profile.
Either way - a single substation solution is easily achievable for something £3m including scrap-value conductor rails, a new rectifier station and some improvements to the SCADA fit.
Its peanuts really.
And whilst the tube exists there will always be scrap value rolling stock available for when the line eventually needs it.
With a new Brading passing loop and single track all the way to both termini I think that would be 2 trains (today four cars max) plus perhaps one shunting.
Indeed, but I just modelled that based on the 6-car platform length.
4-car drops the requirement for new conductor rails in the single-substation solution.
Simples, agreed, but still dangerous with a ground level unshrouded power rail which all the operations and engineering staff on the island have to be trained on, which limits the kinds of minor maintenance work that can be accomplished easily whilst live, and which is just waiting to bite some trespasser, wandering lost child or confused senior one day.
De-electrification will make the line more expensive to operate otherwise and would likely render it even less attractive to the public.
People like to hate on 3rd rail electrification - but for electrifying lightly used branch lines for almost nothing there is nothing better.
EDIT:
And this is before you embrace truly esoteric solutions like a ~£300k 'substation' that contains no actual rectifiers, merely a single contactor and DC circuit breaker used to isolate a bank of supercapacitors that are connected directly across the conductor rail to add stiffness to a supply.
No power electronics or anything and it would easily soak those full power transients.
You might even be able to use a LV substation to support the system in that case because load would never climb above 300-500kW.
