What's the benefit of having security outside the airport?
There are tons of airports that connect to cities by rail transit. Within the recent past, I've been in three of them, PVG, HKG, and KHH. In no case do they put security outside the airport.
For one thing, subway lines have multiple stops. In order to make sure that everyone who's going to the airport has to go through security, you'd want security to occur after the last stop before the airport... and the site after the last stop before the airport is the airport.
For another thing, why would you want to subject people who aren't going to the airport to airport security?
When was the last time you thought "this check-in process would be better if it took up a lot of space in the city center, even though the time requirements wouldn't change"?
Now, rail transit to PVG takes multiple hours. But if you replaced the subway connection with a dedicated rail line with one stop in the city center and one at the airport, that wouldn't help much - that just means that people living nearer to the airport have to travel backwards to the city center, subject to the speed of the subway, so they can take the dedicated airport express to the airport.
The point of doing it this way is that the secure perimeter would extended all the way to the central station. That's the whole idea. Essentially, this urban hub is "the airport" in terms of traveling to it. Think of the actual hub -> board gates/runway connections as like a really long people mover, not just extending a subway line and drawing a new dot.
Trains could travel on a totally grade separated route with strict access controls. Having the last mile part of the train journey controlled by the airport instead of some civic transit authority would allow for a much more reliable system, since they would be insulated from disruption to the larger system, and could also make situation specific choices for optimization.
For instance: Compared to standard transit, many people will be bringing carry on luggage... so probably best to scale everything up maybe 25%...physically make the car wider. Wider doors, wider aisles. Luggage racks everywhere... maybe to a standard width but only have two seats on one side, and luggage rack on the other.
The trouble with this plan is that downtown land is more expensive than rural land. The land area you need to efficiently build the check-in facilities will be similar whether you put it on site or away from the airport. So you'd spend more for the off-site and then still have to build a people mover that provides zero benefit to anyone not going to and from the airport.
It makes far more sense to build the airport far away and add a public rail service to it that can also serve non-airport passengers.
The exact issue under discussion is "We can't build airports where people can get to them because of how big loud and annoying they are". I am proposing a mitigation aka problem solving.
How do you exit? Do you take a train back to city center, or do you have an option to exit near the runways? I wonder if you'd end up with multiple entry points with this model.
You likely would. The simplest example would be a high-speed train that connected two airports (either two existing, or an old and new). You could imagine a high-speed subway that connected JFK to EWR that was about 30 minutes or less. Now you don't have to work out how to get to a specific airport, you get to the closest one.
For people without luggage, the difference is minor; but if you are checking bags it would be a huge advantage to dump your bags as soon as possible.
There are tons of airports that connect to cities by rail transit. Within the recent past, I've been in three of them, PVG, HKG, and KHH. In no case do they put security outside the airport.
For one thing, subway lines have multiple stops. In order to make sure that everyone who's going to the airport has to go through security, you'd want security to occur after the last stop before the airport... and the site after the last stop before the airport is the airport.
For another thing, why would you want to subject people who aren't going to the airport to airport security?
When was the last time you thought "this check-in process would be better if it took up a lot of space in the city center, even though the time requirements wouldn't change"?
Now, rail transit to PVG takes multiple hours. But if you replaced the subway connection with a dedicated rail line with one stop in the city center and one at the airport, that wouldn't help much - that just means that people living nearer to the airport have to travel backwards to the city center, subject to the speed of the subway, so they can take the dedicated airport express to the airport.