I expect that the new stock will be dual-voltage, since all new Third Rail EMUs are built with the capability to work off overhead lines.
After that, its not impossible to imagine overhead electrification of Chester - Birkenhead ...
It is my understanding that existing dual/voltage rolling stock does a switchover between Third Rail and OHLE while stationary (commonly in a station). It is fairly clear why this is the case - it would have involved some serious engineering to provide for switchover at speed.
Third Rail and Overhead electrification each have advantages. Very briefly third rail can have the edge in retrofitting where there are tight clearances - such as retrofitting some tunnels and in some dense urban settings. OHLE elsewhere.
It would like to propose the case that engineering new rolling stock to provide for switchover at speed would be worthwhile, that is to say a paying proposition. Let's look at costs versus benefits.
Costs
involve engineering solutions for automatically raising and lowering pantographs and shoegear. Plus some form of intermediation for the inevitable power surges. Technology makes this kind of thing easier as the years pass. Computer controlled flywheels and trains and purposeful balises spring to mind. A lot of the cost of the present switchover lies in the grounding and isolation work. This could be made a lot cheaper if third rail were converted from 750vDC to AC. Since we are proposing major changes then why not make this change to AC also; actually it may be cheaper overall, certainly in the long run. Maximum supported speed is likely to be an issue also; an engineering compromise must be reached, perhaps in the region of 70mph.
Benefits
Once the price of adopting the new technology has been absorbed there is no longer a time and effort penalty for switching. So because everything happens automatically it becomes no big deal to have short sections of each type of electrification. Having this option available to designers surely lowers the cost of electrification away from the mainlines (where the high cost of rebuilding bridges and tunnels may be justified by the need for high speed).
As to the Southern Railway - it gets to gradually change from Third Rail DC to Third Rail AC, which is probably cheaper in the long run anyway. AC is safer also. As to what voltage (750vAC or 1000vAC or even 530vAC i.e. 750-0-750p-p) is an interesting engineering and financial question with no obvious answer. Not obvious to a retired power engineer anyway.
