I would agree that Pirbright Junction to Southampton Central/Redbridge with dual electrification from Pirbright station to Woking and St Denys to Redbridge, might conceivably make sense as it would allow a linespeed increase to 125mph and facilitate ac hauled intercities and freight on the Crosscountry route. Ideally it would be done at the same time as 25kV electrification and doubling of Salisbury Exeter but I don;t suppose that will happen any time soon.
AC/DC dual systems are very difficult and are only operated where there is no practical alternative, e.g. the short stretch of the Thameslink core between Farringdon and City Thameslink where it gives a turnback facility if trains don't make the switchover correctly. Also there is no need to have dual voltage supplies on the two stretches mentioned above, both being used by dual voltage trains, (convertible), 450/444 to Alton and 450/377 between Portsmouth/West Sussex and Southampton.
It was strongly considered to do the Bournemouth Line electification with 25kV when it was done in the 1960s but they decided against it in the end.
There were no designs of AC/DC rolling stock available in the sixties. Now, for at least the last 10 years, all new stock procured for use in the old BR Southern Region area has been dual voltage, either when delivered or derived from a dual voltage design so that it can be easily converted when required.
The problem is that, other than that, there is nowhere else on the Southern (other than HS1) that there is really a case for going above 100mph (only parts of boat train route 1 and a short section of the Brighton Main line have 100mph now), the rest is 90 max in any case.
Much of the 3rd rail area alignment has been left as is because operating the system is increasingly unreliable above 90mph with a working maximum of 100mph demonstrated in tests. If the supply method restriction was removed, some relatively minor realignment could allow for considerable linespeed uplifts, to 110 mph if not 125mph.
All of the outer areas had a major upgrade only a few years ago with many TP huts replaced with additional substations when the mark 1s were replaced with 37x trains, and most of the signalling/interlockings are fairly modern and won't be replaced for years.the Brighton Main line have 100mph now), the rest is 90 max in any case.
The uprating in recent years was a quick fix to allow the newer generation trains that drew much more power than four EE507 DC motors to run at all. Half of the job was to reinforce the supply just enough to allow only partial crippling of the power curve of the new trains' traction packages.
The fundamental problem is that a 12-car train will need 3000-4000A from a supply system that is limited to around 8000A. So on a busy line, the choice would be put a substation every 2 or 3 blocks or compromise the whole line's service performance by kludges in the trains' software. Also, it's not just the trackside infrastructure that needs replacing, the conductor rails themselves wear down and insulators can become unreliable with heavy use.
The part that needs doing first is the older installations in the London Suburban area where the conversion makes least sense and is most difficult. It would also need wholesale replacement of the suburban stock which is d.c. only and not like the 37x which can be modified to run on either.
You are probably right that the inner area lines are in greater need of replacement, but as you say, there is still quite a lot of DC-only stock still running. I believe that the plan is to start with the outer area where the benefits are far greater, (see above) and keep the metro lines running with high maintenance. The plans to cover the Basingstoke-Southampton stretch as part of the electric spine would also be part of a strategy to work from the country ends of lines and work to the metro areas. By that time, the DC-only stock will have been retired and replaced with things like Aventras and Desiro Citys which would of course be dual voltage.
In the meanwhile they need to extend the third rail to Uckfield and the missing gaps on the Reading Reigate line. I suspect that the current strategy means they wont get electified at all. Ashford Hastings makes sense to do as 25KV as it is principally an extension of HS1 which is already 25kV.
AC electrification is not a problem with AC/DC trains. The benefit of operational flexibility would eventually outweigh the signals immunisation using modern kit and the provision/conversion of a few trains.
Finally, while I am not in a position to know what they are doing, I very much doubt that new signalling installations on the southern, other than in areas with both types of electification like City Thameslink and Ashford International are/will be compatible with a.c. electrification. To do that means making them compatible with both a.c. and d.c. electification which is complicated and very expensive. Visual indication that this is the case can be seen with the signals and their gantries which are not shielded to protect workers from 25kV a.c. (unlike the Midland main line where this has been done since the 1980s on the unelectrified sections north of Bedford.
Newer telecom installations are fibre not copper based for trunk cabling but local trackside cables will still be copper and, again, it would be very expensive to immunise them for a.c. electrification that may never happen. EMC matters are not cheap.
As for signalling, I'm no expert on current practice, but I remember reading something a few years ago about new and updated installations being multi-frequency AC circuits to give both AC & DC traction system immunity. The complications with AC/DC electrification are more to do with ground leakage and safety.
Signalling needs to be upgraded along with the requirements of services, and for a given complexity, I would guess that having developed fairly EMC bomb-proof electronic systems that fibre cabling would not only make installations cheaper, but also less prone to cable theft, (although there have been thefts of fibre - probably by numptys who don't even know what fibre is).
How they must wish that Sir Herbert Walker adopted the LBSCR 6.6kV overhead electification for the Southern instead of ripping it out and replacing it with the LSWR third rail![/QUOTE]
I agree entirely with that. Of course it could have gone the other way on the GER if Holden's Decapod failed it's test and the competition of a tube into NE London (presumably DC) became a reality.
This conversation has become a bit more about infrastructure than rolling stock so I suppose that we must get back on topic.
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But since FCC took over in 2006, it got every 319 that existed and then got the 377s. On the GN side, it re-did the timetable massively and secured more stock - as well as platform lengthening and power upgrades for more 12 car services. More 313s were obtained so almost all peak services are 6-car.
So TOCs do try and put on extra capacity in the peaks, but many are at the (current) limit without building more trains or upgrading infrastructure.
Once you're at the limit, you might as well have people not travelling.. although most people will soon realise that the alternatives are not actually better (driving into central London?) and will work out their own way to travel in more comfort.
These guides to the busiest services are certainly helpful in this regard, but any commuter should have already worked all of this out in a relatively short space of time anyway.
The big problem with securing a shed load of stock for the peaks, lengthening every platform, upgrading the power and signalling to let more trains run together, is that you've now got a lot of money spent on just a few hours per day.. and then empty trains for the rest of the time.
If people could stagger their travel times, spreading the load, it would help massively and everyone would be happier.
Maybe they could reduce the season ticket subsidy for peak-hour travel.
