How to you account for 17k potential passengers ? How many coaches is that and what are the pathways ? Do you account for 17k passengers or a percentage based on potential alternative travel options ? What about your regular passenger flows ?
It'll actually be 34,000 passengers - 17,000 off the ships; another 17,000 on the ships.
Even if only a small proportion of those opt for rail, it's a substantial extra volume and as others have said a lot of them will be dispersing via London so there probably is a case to look again at running boat trains. The stock can bring embarking passengers to Southampton and return with the disembarking passengers.
The real difficulty is going to be that it is not a regular flow; there will be some days like this but other days with no cruise ships in at all. That then returns us to the old headache of stock utilisation although it wouldn't be impossible to negotiate with the various shipping companies to try to spread the loadings out ( which would help smooth things for the port as well ). There is a long lead time on that sort of negotiation, though - ship schedules are planned at least three years ahead in most cases.
All this means there is no short term solution, and a longer term strategy is going to require multiple parties to negotiate together - the railway, the port, the cruise lines... and then there is the question of reliabilty and whether or not a through ticketing option can be included within the cruise fare. Get that aspect sorted and with a return to central London luggage check in to make it more seamless for the passengers, it'll probably work. It'll still take a long time to do, though - and would work much better with the lines into the port itself rather than Southampton Central being used.
