[rplnt]

Prague has/had over 50 km. Was used up until early 00s. Loads of it still exists, it's the last surviving municipal tube system, though not functional at the moment.

https://en.wikipedia.org/wiki/Prague_pneumatic_post

[jaclaz]

A page dedicated to these (city wide) pneumatic networks:

http://www.douglas-self.com/MUSEUM/COMMS/pneumess/pneumess.h...