If money were no object I would modernise the railway along the entire South Coast at least as far west as Southampton and at least as far east as Ashford to enable intercity style services as well as express commuter services.
Obviously signalling and track would have to be updgraded throughout to allow a decent linespeed.
Some kind of hybrid third rail DC/diesel express traction would be nice assuming 25kV electrification never happens on the rest of Southern which would share the tracks - or could third rail and OLE coexist?
Hastings to Ashford would have to be electrified in some form in any case.
I suspect twin track would continue to be adequate between Brighton and Ashford, though some decent length passing loops and additional platforms at intermediate stations would be needed. It would be nice to do away with some of the lesser-used stops.
The Willingdon Chord at Eastbourne would have to be reinstated unless someone comes up with a speedy way to get trains in and out of Eastbourne.
Brighton station does not seem well equipped to accommodate express east-west capacity. It would be necessary to build a new grade-separated set of East-West tracks and platforms over/under Brighton station or the station throat. Getting grade separation on the approaches would be tricky. The viaduct from the Lewes line is setup to put trains into the station mouth, not across it. However, whether you went up and over the station throat or down and under the station itself, there seems to be room to continue to the west with new tunnels through the chalk, rejoining the existing formation before Hove.
From Brighton westwards the local stations come thick and fast. Luckily the right of way is generous here, and it looks like there is ample room for 4-tracking. Several of these well maintained but uninteresting stations would have to be rebuilt to accommodate a pair of through express lines.
There are a dozen or so level crossings between Hove and Ford. Surprisingly, an aerial view on Google maps suggests that demolition and replacement of the stations and one or two lesser commercial buildings would allow most of them to be dealt with by road over rail bridges. Portslade might be tricky, but in the worst, most expensive case the railway could always be elevated here to cross the road.
If capacity dictated four tracking all the way to Ford, then of course a new viaduct alongside the existing one at Shoreham would be needed. Getting quickly over Arundel Junction would suggest grade separation and maybe some clever realignment.
West of Ford my recollection of the line and how it is used are hazier still, but there might be a case to put passing loops in at the stations rather than fourtracking throughout, as I get the impression that the line is relatively quiet until you get to Portsmouth and Southampton when it probably all gets a bit tricky again.
So what would it cost? Would 5 billion do it?
For that you get 130 miles of 125mph capable double track railway integrated with the local stopping service to offer both intercity and express commuter services along a route that is currently painfully slow and relatively underused. You would be serving a very densely populated corridor, surrounded by a sensitive landscape where people currently struggle with horribly overcrowded and badly planned roads. You would be connecting the channel tunnel with the cities of Southampton, Portsmouth and Brighton, not to mention the likes of Hastings, Worthing and Chichester, and less directly Eastbourne.
Historically, it seems that the South Coast has never been economically integrated with itself, being made up of ports and holiday resorts, both of which were built with transport infrastructure designed to reach London and other inland destinations. Yet the reality of employment, education, healthcare, etc means that the there is plenty of need for people living on the south coast to travel to other places on the south coast, sometimes over considerable distances.
Would there be any case for freight? Gauge clearance work would not seem too far out of scope. Of course there are ports all along the route which might produce a role for rail transhipment to consolidate goods from different ports for onward shipping....?
Of course, why stop at Southampton.... why not continue all the way west? The distances involved are considerable, England is as wide at this point as it is tall.
