Les, I was thinking of something similar to your 40 mm pipe, but use it in a similar way to the catenary wire(mentioned by Bazyle), propel a cassete with compressed air, this system is used in places like Hospitals for example, for sending samples to the lab etc. I can remember both systems being used up until the 1960s in department stores in Dunedin (NZ). One method whoosh, message gone, or received, second method clatter, zip, bang, message sent/received.
Theres always the old fashoned system my Grandfather had in his factory, long tubes, first you blow down the tube, and a whistle blows, person at other end removes the whistle, and the tube becomes a speaking tube, I'v know idea how effective it was, but Grandpa loved gadgets. Ian S C