There is no way the 415V local supply network could supply a traction load.
Then why do DNOs quote for 300kVA 415V supplies as a matter of course then?
For a start you would need to run a heavy duty cable back to the substation as the cables in an average street would overheat with anything more than 10s of KWs extra load.
Which again, is done as a matter of course by DNOs - who operate enormous LV distribution systems already.
TThen the substation which often have a continuous rating of a few hundred KWs would need upgrading to cope with vast increase in load.
So the unit substation would be replaced by a slightly larger unit substation?
Finally as domestic supply substations are typically spaced about 500M apart and you could only put one traction supply per substation area then you would be lucky if you could supply power at even 1MW per KM of track - barely enough to run a very light tramway service.
1MW per km of track is a ludicrously heavy traction load for most areas?
A twin track railway with 1MW per route kilometre would have Class 319s at 2km spacings in both directions, or Class 444s at 4km spacings in both directions.
The lines that currently remain unelectrified would have traffic densities far lower than that!
Agreed - completely, plenty of dreaming going on.
The current minimum power assumption per train is 2.7MW, hence the capacity of substation is dependant on the density of trains in the supply area (with larger supply areas being better for regeneration but worse for everything else). The maximum current flow at any point in the circuit is limited to 9kA so nominally 6.75MW which is enough to support 2x 12 car 700s on max 3rd rail performance setting.
So which currently unelectrified railways would require performance of multiple 12 car 700s at maximum power output?
Also that implies single ended feeding! Otherwise the current rating is potentially up to 18kA, but slightly less as the circuits won't perfectly share the load.
A far more reasonable load is a single unit Class 319 or a Class 444.
Maybe a double Class 444 on the West of England main line but they are only hourly (half hourly Salibsury-Basingstoke)!
In reality the cost estimate from the DNO for doing that would make it cheaper to install your own 33kV cable which is why L&SWR + SER / SR / BR / RT / NR have always done that!
Southern Railway/Southern Region did not always use a 33kV distribution system they owned, in fact they later abandoned that concept in favour of grid substations connected to whatever supply was most convenient - which is how we ended up with 11kV connected substations.
This is just tkaing that concept to it's logical conclusion.
The SOuthern Region distribution system exists as a historical hangover from before the era of widespread high power grid systems existing.
The biggest 3rd rail supplies are virtually effectively fed direct from Nat Grid at New Cross (ex Gas Works) / Plough Lane with rail exclusive SGTs and minimal DNO involvement.
So how is this relevant to the idea of electrifying new areas?
No areas that are not currently electrified are going to see any traffic densities like that.
All this dancing on the head of a pin to find a way to build more 3rd rail. For anything other than genuine metro lines with very controlled track access, the new 3rd rail ship has sailed. In terms of safety, energy use and performance, 3rd rail is a 100 year old solution that doesn't fit the requirements of a modern railway.
And 25kV is a 60 year old solution that doesn't fit he requirements of a modern railway?
It is hilariously expensive
Just like the solution to the NI border there may be some whizzo tech solutions in the pipeline that address one or more of the ORR issues, but in the meantime, OLE is the proven answer for anything being planned now.
The only thing OLE is a proven solution for is burning vast sums of public money.
The only thing insisting on 25kV for new installations will achieve is
ensuring there are no more installations
New and replacement bridges are generally built to current standards so when those over 3rd rail routes are time expired they will gradually become more compatible with the requirements for OLE.
Except all the work of 30 years was blown up because the ORR decided it wanted a proper railway that had proper clearances in yet another example of mad gold plating.
EDIT:
For example, it takes roughly 40 minutes to travel between Basingstoke and Salisbury at the present time, assuming we take no credit for replacing 159s with 444s (as a traction exemplar for what a long distance third rail unit might look like), we would have at most two trains in each direction at any time.
Four trains total in the scheme, composed of 2 units each so 8 444s.
That is a maximum traction load, assuming all units take full power at once, of something like 16MW.
16MWe on the roughly ~52km between the SWML junction and Salisbury.
So that is a
maximum possible traction load of ~300kW/route-km.
And that takes no credit for support from substations further west, or timetabling, or electronic power restrictions if the voltage on the rail sags (which units are required to do according to the specification), or any diversity at all.
Class 444s have such better performance than Express Sprinters that its entirely possible the number in trains in transit will never reach 4.