That sounds like it poses a ton of engineering challenges to solve a problem that doesn't exist. Like what if you could just get off the plane and go straight to a bus which would take you to a train station? Of course you'd have to sort out the luggage so it'd probably make more sense to have everyone go through the airport anyway. At that point, if there's a train station at the airport, you can just walk to it - or if you're not going to the train, you can go wherever you need to (while also having a chance to eat, use the bathroom, and stretch).

Planes and trains are made of different materials and are designed in different ways. Time that the plane is travelling to and from the station is time it's not serving it's core function, and there's new sources of wear and tear that have to be planned for. The only real advantage to all this is that the passengers "get" to spend more time seated at once, which nobody really asked for.