The best example is the tube system in the capital of Czech Republic. The Prague pneumatic post (Pražská potrubní pošta) is the world's last preserved municipal pneumatic post system. It is an underground system of metal tubes under the wider centre of Prague, totaling about 55 kilometres (34 mi) in length. Known as the Old Lady of Czech telecommunications, the system started service in 1889 and remained in use by the government, banks and the media until it was rendered inoperative by the August 2002 European floods. The current owner, Telefónica O2 Czech Republic gradually is repairing and conserving the system; due to limited funding the system remained inoperative as of March 2007.
The system was established for those desiring to send a document fast. The document would be taken to the post office and rolled up into a metal capsule. The clerk would then drop the metal capsule down a hatch leading to a predestined location. After the clerk pressed a button, the capsule would be moved by compressed air along a network of tubes beneath the pavement.
The main growth of the network dates to the economically prosperous era of 1927–1932. In those years, new lanes were constructed and tens of thousands of capsules transported per month.
During the Prague Uprising (1945) the pneumatic post played a role in supplying the besieged building of the Czech radio. Below: a "tube" postcard mailed in occupied Prague, 1942.
In the late 1990s, the system was used by over 20 subscribers and operating at a loss, so kept rather for prestigious reasons. The traffic weakened gradually and the 2002 floods seriously damaged it, flooding 5 of the 11 underground engine rooms.
The lanes consist of steel pipes of 65 mm bore and wall thickness of 2.5–3 mm. The pipes are connected with tight couplers 14 cm long to ensure perfect coaxial alignment and then welded together, ensuring air-tightness. To prevent dispersion current from causing excess corrosion, ceramic insulators are inserted between the pipe segments at some places. Pipes buried underground are protected from the outside by a layer of fiberglass, wound around at increased temperature and coated with hot asphalt. The pipeline is typically buried under the Prague sidewalks 80–120 cm deep. Inside buildings and in the Prague trunk conduit network the pipes simply are coated with anti-corrosive paint.
The minimum bend radius is 250 cm for underground pipes, but 300 cm is the most commonly used radius. Inside buildings a bend radius as low as 200 cm is allowed. The bends are made of special annealed pipes at normal temperature, using a custom-made bender. A signaling cable is laid along with the pipe, enabling communication with the track components. The lane segments are equipped with dumb wells, where the pipeline can be opened and inspected, or a stuck capsule removed. For this purpose a heavier capsule can be sent at a pressure of up to 30 atm, knocking the stuck capsule out.
The current lane state was indicated by indicator lights on the lane's controller. Up to 10 packages in 30-second intervals could be sent on the same lane at once, although this was rarely used in practice.
The system uses aluminium capsules measuring 48 mm in outer-diameter and 200 mm in length. On the rear end they are fitted with a plastic circlet, preventing friction against walls of the pipe and a soft plastic skirt, sealing air behind the capsule. The diameter of the rear circlet is 57 mm. The remaining 8 millimeters of the bore are sealed just by the skirt, allowing for excellent airtightness and low friction at the same time.
Each lane is equipped with a dedicated propulsion unit, consisting of an electrically-powered air pump. One pump can service at most 3 kilometers of pipeline, so it's necessary to use several pumps on longer lanes.
The pumps must be reversible, creating either pressure or vacuum. The pumps are connected to the pipes with tee-fittings. On both sides of the tee the pipe is equipped with switches activated by a passing capsule.
The Prague pneumatic post network consists of five main lanes arranged in a star topology, fitted with switches and concentrators, and of subscriber's lines. Originally there were 16 subscriber's lines, but only 7 have been preserved up till today. A total of 24 pneumatic post stations remain today. The network crosses the river Vltava in three spots making use of bridges.
All the lanes converge to the main post office in Jindřišská st. Here all the packages were carefully recorded and from here the network was controlled and monitored. This is also the place where the packages were forwarded from one lane to another. The capsule was picked up from a receiving pocket by a member of the staff, recorded and inserted into the inlet of another lane.