Great Western Coffee Shop

And here is some good news for all those celebrating the end of the Portsmouth engineering works debacle (link below.)
http://www.rmtbristol.org.uk/2007/10/biggest_shake_up_in_southampto.html#more

Quotes :

"The work, due to start early next year and finish by 2011, will include improvements to the tunnel linking Southampton port and station with the rest of the network. The growth of rail freight has been hampered by the use of the larger containers, which are too big for certain sections of the network when fitted to standard rail wagons.

The project is likely to mean misery in the short term for commuters on road and rail,with disruption unavoidable.

Work to widen the major tunnel that runs underneath Southampton city centre is set to cause long-running disruption to services and to motorists."

If you think that disruption to Portsmouth - Cardiff services is bad enough under normal circumstances , just wait until this baby gets underway.

Further quote from Network Rail :

"We will certainly bear in mind any potential disruption the work could cause, and we will do our best to minimise that disruption."

Now where have I heard that before?

No problem, re-route Portsmouth - Cardiff services via Chadlers Ford and Eastleigh for the duration with a shuttle Southampton - Romsey.

BR did extensive work on the tunnel in the early 80s. For many months there was single line working, but with interlaced tracks as opposed to a true single line. They managed to maintain a pretty reasonable service, with the interlaced line occupied about 100% of the time. Whether H&S would permit such an arrangment these days is highly doubtful. At weekends the Chandlers Ford line and Laverstock Curve (Salisbury) were used for diversions. The latter was reinstated especially for the purpose.

I would assume that FGW would go via Chandlers Ford and Botley and Voyagers via Soton, Romsey, LAverstock Curve, Andover

And the Freightliners? Melksham by any chance?

Would be a good guess, or B&H as they did during the Basingstoke Blockade the other month

or via the Berks and Hants line via Newbury.

I take it that the new SWT Romsey - Eastleigh - Southampton - Salisbury service would have to be suspended (at least in part) and instead have the Portsmouth - Cardiff trains call hourly at Chandlers Ford. Also , what would be the likely arrangement for Waterloo - Bournemouth/Weymouth services and non - FGW services coming into Southampton from the Portsmouth direction if John R's suggestion isnt possible?

We are only hazzarding at guesses here. If the work entails digging out the trackbed to lower the line then a full blockade with bustitution Eastleigh - Southampton. If the work means cut and covering cant see services being allowed to continue whilst 'debris' is being removed overhead but then a full blockade also.

Then what buildings stand over the tunnel route which would preclude cut and cover.

Probably through services truncated at Eastleigh and resummed at Southampton.

I should imagine they will use some sense and just run a 'regular' service from Eastleigh-Soton like they did from Pompey-Fareham last week

When it last happened the good old rattling REP+TC formations ruled the route, so it was dead easy to keep the 33s on (instead of detaching at Bournmouth) and then quickly reversing at Soton, Romsey, and Eastleigh (for the 92 headcode semi-fasts), or running fast to Basingstoke via Laverstock (for the 91 headcode fasts that once ran non stop from Soton to Waterloo every hour - sorry, I'm getting nostalgic again now).

Neither option would be possible with the current stock.  I guess it depends how long a blockade might be as to what options will be considered. There's a stock of Adelantes that could provide a service via Laverstock for a period.

My recollection is that cut and cover is not possible.