I absolutely agree that it would be costly and an engineering challenge, but certainly not impossible.
The Copenhagen airport (waterfront location) is serviced by both regional rail and direct service to their central station, for example. You can get to a number of points throughout Denmark without a transfer.
Charles de Gaulle (Paris) (not waterfront obviously) has exceptional train service including both regional rail and TGV service to points throughout France. That was not cheap to implement, but they wanted to make it happen. (By the way, this arrangement not only reduces private vehicle usage, it also reduces the need for regional air flights across France -- an even bigger polluter per passenger!)
And that is kind of the point, you have to want to make mass transit work effectively and efficiently in order to have that happen. If our society continues to view transit as something for poor people, all we get are worn out trains and busses in dingy, century old tunnels.