Hamburg built a system with a pipe diameter of 45cm in the 1960ies but the system was scrapped soon after. It would be a godsend nowadays. See Großrohrpost on t

Hamburg built a system with a pipe diameter of 45cm in the 1960ies but the system was scrapped soon after. It would be a godsend nowadays. See Großrohrpost on this page https://de.m.wikipedia.org/wiki/Rohrpost_in_Hamburg https://de.m.wikipedia.org/wiki/Rohrpost_in_Hamburg (sadly no English translation exists)