electric_post_tube

use powered electric coils to send small vacuum tube sized mail containers with permanent magnets attached around individual buildings and eventually around a city. Inspired by see link [https://www.youtube.com/watch? v=J9b0J29OzAU] except powering the coils rather than the vehicle so you don't have to lug the batteries around with the vehicles (just use permanent magnets instead) and can just send a canister of standard vacuum tube size. You could run the coils around irrigation pvc pipe for an initial like prototype model, because its got holes to prevent air cushions (rather than going airtight, which was the main issue with vacuum tube systems, maintaining a vacuum over long distances is hard so it never scaled). The pvc would provide a smooth surface for the cannister to move through and the coils could be outside of that. You'd want to then protect the coils thermally and electrically from melting the pvc and from melting / conducting through anything outside. and then your issues are, supplying power (easy, get it from building or municipal power), switching (harder. I'd try and do most of it through y-junctions and use electromagnets to pull the 'packets' one way or another) and routing packets (harder. I'd start with something inspired by how packets are routed on the internet, so they can route around blocked places).