The research I undertook (it seems many years ago now) was based on the idea that comparing air travel with overnight rail was to look at the total journey experience. Typically city to city an air traveller has to travel out to an airport (with some time and cost), hang around in the airport to go through the various security checks and boarding pass queues and deliver up their stored luggage (possibly paying extra for the service) before being shoe-horned into a potential flying bomb for a relatively short duration flight. At the other end there will be the usual passport checks, luggage reclamation (for those with stored luggage) before finding transport into the city of their choice. For someone wishing to arrive for an early meeting in a distant city the choice is often an unearthly departure from home or a hotel close to the airport. So to compare a 2 hr flight with a 10-12hr train journey is to hide much of the time (and cost) of getting to and from airports, as well as the limit on the earliest flights.
The train service used in the research was based on the double-deck CityNightLine cars, fixed in 100m or 200m high-speed multiple units to run in multiple rakes to various destinations, splitting and coupling as need be to reduce the number of route paths to be purchased. The routes chosen were based on popular destinations (from London) which were about a night's sleep away - that is 7-12hrs total journey time. One of the benefits of railways is that can pick up and set down passengers along a route.
It was true that most of the people replying to the questionairre wanted to know the potential cost of the rail service, and at the time (~2005) we were working on around £100 for a single journey of up to 800km. The service still showed a great deal of support, simply because the costs were comparable with most flights of the same distance, based on the city centre to city centre costings. Additionally attractive was the fact that you could have an early arrival without the pain and (physical) cost of very early starts. Those who replied and showed an environmental interest were attracted by the fact that per passenger mile a plane uses around 10 times more energy than a train. Economically, during times when avaiation fuel is ever more expensive and with the introduction of carbon taxes the economic reasons for taking the train become stronger.
However, all this depended on delivering a reliable, comfortable and cost-effective service. It was always a joke that when conducting this research we wished that the Channel Tunnel had connected with any other country except France, as that is where we came against the greatest difficulty politically, and where the rail unions are most resistant to any change (and strike more than anyone else).
In the end, it was the UK government that provided the least flexibilty. We had tackled all the technical challenges with the rolling stock, including traction, signalling, infrastructure limitations etc. We also seem to make the numbers work (though we were dealing with a good deal of soft data). But it was the security arrangements that UK authorities were unwilling to change. By insisting that every passenger using the service towards the UK was to have a passport check and security scan, and be held in a secure waiting area before boarding the train meant that every station we wished to serve in Europe (and potentially it could have been any station - if our charter service was to have gone ahead) would have to have built all the paraphenalia that is found at St Pancras, Bruxelles, Paris Nord, Avignon and Bourg St Maurice, including X-ray scanners, passport booths, secure platforms, even little jails, to comply with the Eurostar agreement.
At that point we gave up. Bar the UK security restrictions we felt it could have been a wonderful, viable service. We tried introducing the idea that security and passport checks could be carried out on the train prior to arriving at the Channel Tunnel entrance (as it is this strategic piece of infrastructure that is ostensibly being protected from rogue train travellers, as well as being the border), but that was discounted as 'not being the same as' the current security arrangements. We investigated near future passive and active scanning devices, some of which could have been incorporated into the train doors, but again, not being a proven technology was discounted. Depressing, but that is where we are. Perhaps I will try again, but right now the economic situation means that as a sustainabilty and engineering consultant I am getting no work, and with a family to support it is not easy to lobby the individuals that could make a difference.