[Retric]

There have been small scale versions of this used in major cities in the past. The primary limitation has been package routing, but that should be a reasonably simple problem now days. Realistically outside major city’s or industrial areas, building tunnels is unlikely to be cost effective. However, in a major metropolitan area it may be cost effective depending on what rates you can charge.

PS: Same day currier companies already use the subway system; they have a "mole" that stays in the subway and hands packages to curriers at each station.

[chopsueyar]

The tunnels were then used to flood the basements of unsuspecting businesses.

http://en.wikipedia.org/wiki/Chicago_Tunnel_Company

[sorbus]

Your wording is somewhat misleading, as the flooding was entirely accidental in all of the incidents mentioned.

[chopsueyar]

I apologize for making you read the wikipedia article.

;)