If you're serving Disneyland, you'd do it as it's own train as the potential market for that will be very clustered around check-in and check-out times for the hotels.
That’s not what the late lamented Disney trains did though.
Bordeaux was being talked about very strongly
I wouldn’t say that. HS1 were touting for business. Eurostar had a little think about it, possibly.
I wonder what sort of demand there would be in a hypothetical world where the UK was still in the EU and had joined the Schengen Area? If there was no need to worry about immigration rules, would there be a much greater range of destinations available, or would it still be uncompetitive with flying?
Yes, there would be a much greater range of destinations. Probably Hamburg, Frankfurt (maybe even Munich), Zurich, Milan, Geneva, Nice, Barcelona (maybe Madrid). Not having any form of security or border control would just make them regular trains that could pick up / drop off passengers anywhere.
Just one example - there are a number of services that start in Lille before heading to the south of France. It would be relatively low cost to extend those to start / terminate at St Pancras, where there is a much bigger market.
Two main drivers of the high prices right now are:
-limited capacity at St.Pancras
-high debts accrued during the pandemic
By far the biggest driver is that the market will bear it, and that is because it is (for most people) quicker and more convenient than flying to Paris.
The second biggest driver is that the cost base for running a railway is substantially more than for running an airline on a seat /km basis, particularly for longer Rail journeys
If it was completely unchecked then checkin could easily be 15-20 minutes before departure, which knocks nearly an hour off the time needed for starters.
Reasonable to assume that with no border control or security, you wouldn’t need check in. You don’t check in at
Basel for the Hamburg train, you just get on.