I suspect you’re getting downvoted for the pneumatic tube sentence, because tube systems like that have proven to be way too inflexible for a rapidly changing city. And expensive. And hard to maintain.
But I agree with the rest of your comment.
But I agree with the rest of your comment.
Also looking at projects like the hyperloop, I'm wondering if a smaller version linking cities would make sense. We already have infrastructure for transporting oil and natural gas through very long pipelines. How about actual packages? If the hyperloop can make sense, I feel like inter-city mini vacuum train probably can as well, and linking two cities together doesn't require any great flexibility.