As I noted, at DLD Lufthansa had a contest (which I won) for suggestions on how to innovate to compete with trains. They set the time horizon out 15 years, which really means a lot is possible, so while I mostly threw in ideas from this blog which are short term, I put in some longer term ones too.

One was the equivalent of "multi modal transport." To do this, you would build new short-haul planes which consisted of an empty shell, like the cargo planes you have seen where the nose hinges up, and cargo modules are slid in on rails. This would be combine with "passenger modules" which can slide into the shell, and which can also slide into a special rail car. There might be one module on a plane, though it is also possible to have several.

Passengers would board a train normally at the train station. Then, as the train moved to the airport, they could move to the passenger module car. They would place their luggage onto a belt to put it down low into the luggage module (under the passenger module) or be assisted by a porter. They would enter the passenger module, stow their carry-ons and otherwise get ready in their seat. By the time the train got to the airport, all passengers would be in their seats, belted and ready.

The train would split up into different cars if there were several flights on it, and each would move to a terminus where the plane-shell was waiting. Yet to be invented technology would laser-align the train and the parked shell in advance, and then the passenger module would slip into the aircraft hull on special rails. Connecting passengers could board the train at the airport before it moves to the hull, and their bags could be loaded into the bottom the standard way. (Though this is for short-haul flights, so there may not be connecting passengers.) An automated system would connect power, data and air venting on the passenger modules. Water/sewage would be self-contained and processed at the train station. Catering would probably be handled there too.

The nose would come down, the pilots board via their own door and takeoff would begin shortly.

Because the plane would be a modern fly-by-wire system, the pilots would have done all their pre-flight checks from a ground-based module, so they don't have to wait to do more than check out the cockpit after it lowers.

Now you will note I didn't talk about security. That's because perhaps there isn't much, if any. The luggage modules and the passenger modules are built so that small bombs can explode inside, and will deform, but not rupture the module. The fly-by-wire planes have several redundant power and data channels (encrypted of course) so that even a rupturing explosion will not make the plane unairworthy. If there are multiple modules, a bomb in one would not hurt those in the others. The cockpit is completely isolated from the passenger modules, so no passenger can take control of the plane.

In other words, you don't need any more security than a train or a bus. Yes, a bomb could kill passengers, which would be horrible, but no more than it could in any other crowded place or form of transportation. There would be some security, to screen bags for large bombs, but that might be it.

If the world isn't ready for that, security can be done at the train station, or even better, on the train as it moves to the airport. As I've described elsewhere, security staff with wands and x-ray machines could move through the standard train cars, clearing people as they go through. The only downside of this is they have nothing to do on the trip back.

If checked luggage needs more scrutiny, the use of train stations rather than airports allows an interesting approach -- sending your checked luggage ahead by taxi or other delivery service to the train station. This works because for many, train stations are close by, in easy cheap range of taxis or even personal drop-off. For many flyers, their checked luggage is actually ready well in advance of the flight, and so it can be sent early for any extra scrutiny or processing it needs.

Indeed, it's not out the question that even carry-on luggage could be sent that way, and even placed in the bin for one's assigned seat in advance in the passenger module. For this the modules would have to be cheap enough that they can have extras so there is more time with them, and of course it only works for certain carry-ons. My carry-on bags usually contain both fragile items (my cameras and computer) and also "absolutely must not be lost" items I refuse to let out of my control.

Since trains have a maximum width, this approach only works for narrow planes, probably configured as 2 and 2 or 3 and 2, for short-haul flights. It's not impossible to imagine widebody planes with two modules that are the left and right half, though these might also get too tall for the rails unless the luggage and passenger modules are kept independently. However, the main goal of this would be short-haul flights, where the time saved by such a system is really meaningful.

Done right, it could be as simple as getting on the train, zooming to the airport and perhaps just 5-10 minutes for the transfer, taxi and takeoff. Such a system makes it impossible for trains to compete with planes on the coveted "downtown to downtown time" only any route a plane would fly. Now any train station can be an air travel destination.

Note:

• This does cost extra weight of course, for the two shells and the rails and a few other redundant items, as well as the fancy nose.
• I expect the main hull would be pressurized, and the passenger module not, allowing it to be simpler and lighter. But it's possible to reverse that, eliminating the need for ducts to the main hull air system.
• Emergency exits now have two doors (one on module, one on outer hull) adding some complexity.
• You do lose a few inches of coveted width, but your real limit for the passenger module is the width of the rail car, anyway.
• Flight crew can't get into the cabin or get food from it, but this is OK for short haul.

It's also possible to imagine a plane designed to go on a train more fully. This could be a vehicle where the wing is able to rotate 90 degrees, the way some car/plane combos were designed. The tail still presents a problem. One could perhaps build a fuselage which is able to be mounted on a wings/tail base. This means the whole fuselage comes on the train, which is fine because it's nice and streamlined. I think the connection here is more complex, though.