This section is an introduction to what capsule pipelines are, what they do, and how they work.
Capsule pipeline transport can be defined as the movement of goods or passengers through an enclosed length of tube, in membranes which enclose the cargo.
No standard definition exists. Encyclopaedia Britannica (based on Liu) is more exacting, in that capsules convey freight, and be propelled by a fluid . The terms 'tube transportation' and 'tube freight' are broadly interchangeable with 'capsule pipeline'. Tube transportation has been defined as a class of transportation system in which close fitting capsules or trains of capsules move through tubes between terminals .
Capsule pipelines may be considered a sub-class of freight pipelines, which also include pneumatic conveying (material suspended in a gas) and slurrying (material mixed with a liquid). Capsule pipelines are least widely used of all freight pipelines.
Capsule pipelines might be differentiated from underground railways and most underground automated freight systems by the fact that in a capsule pipeline system the source of motive power is not mounted onboard the vehicle (the capsule), where as in a railway system it is normally mounted onboard the vehicle (the train). There are further grey areas involving vehicles that are sometimes designed to run within tubes, but where the core underlying technology does not require the vehicle to be contained in a tube - there is a common area of overlap here with magnetic-levitation based projects.
In practice the division between capsule pipelines and railways, automated freight or similar systems that merely run in tunnels, is difficult to accurately define.
This site tends to be focused on a core that most people would agree are 'capsule pipelines'. These are generally the areas about which there is least information available elsewhere. Underground automated freight systems and underground railways are therefore excluded from the detailed texts on this site, although you may find passing references to them.
Comparison to Other Transport Systems
A capsule pipeline system is similar to many other land based transport systems. It involves a link (the pipeline) between terminals (where the item being transported enters and exits the system). A vehicle (the capsule, containing the item to be transported) moves along the link. This is shown in the figure. At this conceptual level capsule pipelines are the similar to road and rail systems.
Conventional pipelines transport either a fluid (a liquid or gas), or transport a material suspended in a fluid. Conventional pipelines are an important mode of transport, although since they are normally not visible, and do not cause many of the problems associated with other modes of transport, they tend to be ignored. Water, gas, sewage and oil are commonly transported by pipeline. Many solids can be slurried (mixed with a liquid), the material transported in suspension and then separated from the liquid on arrival at its destination. Certain solids can be suspended and transported in gas (normally air): this is referred to as pneumatic conveying.
Capsule pipeline systems differ from conventional pipelines. The item being transported is placed within a capsule. The capsule is then propelled within the pipeline. In most capsule pipelines the capsule is propelled by the force of a fluid acting on the capsule. The nature of this fluid is used to sub-define capsule pipelines. Where air (or a vacuum - absence of air) is used, the term Pneumatic Capsule Pipeline (PCP) is applied. Where water is used, Hydraulic Capsule Pipeline (HCP) is the term applied.
Capsule pipelines may be applied to both passenger and freight movements. Most historic, existing and proposed applications are for freight transport.
Small diameter pneumatic capsule pipelines were used to transport telegrams and telegraph messages in Western Europe and North America from around 1850. Most of these systems fell out of use in the second half of the 20th century. Small diameter systems are still used to transfer small freight items over short distances. Common applications involve moving cash from supermarket tills to backroom offices.
Larger diameter pneumatic systems were developed by the Victorians as an alternative to underground railways, carrying freight and passengers underground in cities. During the 1960s and 1970s large diameter pneumatic systems were further developed as a potential technology for high speed ground transportation. Recent proposed applications range from the distribution of household goods, mail or refuse, to inter-city or inter-continental freight movement. Movement of passengers might also be possible.
New technology and greater environmental awareness are making the economics of capsule pipelines more attractive. Capsule pipelines may provide a viable alternative to many existing road and rail based transport movements. The difficulty in financing and developing extensive networks means the most successful proposals and applications are in niche markets, such as cash movements within supermarkets, or movement of coal. Potential exists for capsule pipelines to be applied to almost any earth-based transport movement.
Capsule pipeline systems pick-and-mix different technology, depending on what the system is designed to do. There is no single method or approach, although some techniques are common to many systems.
Pneumatic Capsule Pipelines (PCP)
Simple PCPs follow conventional fluid mechanics principles. Air is blown down and / or extracted from the pipeline, propelling the capsule along the pipe. Both ends of the pipeline are sealed during transport, allowing the air behind the capsule to be above atmospheric pressure, and/or air in front of the capsule to be below atmospheric pressure. Simple PCPs involve a limited number of capsules in the system at any one time (normally just one). This reflects the inefficiency of creating sufficient pressures to propel multiple capsules, and the difficulty in retrieving one capsule from the end of the pipeline while a second capsule was being propelled.
Modern PCP large diameter systems utilise through flow booster pumps, also known as jet pump injectors. These create the pressure differentials required to propel multiple capsules through a pipeline, while allowing both terminals at atmospheric pressure. This is done by placing a booster pump midway along the pipeline, and designing it in such a way that capsules can pass through the pump.  gives a technical explanation of theory associated with this. On long lengths of pipeline multiple booster pumps can be placed at regular intervals to allow many capsules to be propelled simultaneous without significant inefficiencies in the system. This type of PCP is still restricted in the number of capsules which can be conveyed at any one time, unless booster pumps are placed at very regular intervals. Uneven flow (irregular capsule movements) or very high speeds tend to require the system to be designed in a way which makes inefficient use of energy. The best technical/mathematical summary of research into large diameter PCPs is .
Small diameter capsules rely on a felt or plastic ring at each end of the capsule to provide both a seal between the capsule and the pipe wall, and to reduce friction of the capsule against the pipe wall. In larger diameter systems, or where heavier loads are conveyed alternative means of reducing this friction are required. Early PCP systems tended to mount capsules on railway tracks within the pipeline. Later PCP systems used clusters of wheels mounted at each end of the capsule. The wheels face onto the pipeline wall, and are arranged at even intervals around the circumference of the capsule. The capsule normally pivots between its two clusters of wheels, allowing the wheel clusters to move freely in the pipe without up-turning the contents of the capsule. Alternative means of reducing friction have been proposed (for example by using opposing magnets on the capsule and pipeline to suspend the capsule in mid-air within the pipeline ), but none have been implemented.
Hydraulic Capsule Pipelines (HCP)
In HCP, capsules are conveyed in a flow of water along a pipe. At low water speed the capsule slides along the floor of the pipe, however once the speed of flow is sufficiently high lift is generated (similar to that in an aircraft) and the capsule becomes waterborne. Once this is achieved transport of the capsule only required 10-30% more energy than would be required to move the water alone. HCP has spawned the concept of Coal Log Pipelines (CLP). These utilise the same principles as HCP except the capsule is made up of a 'log' of the cargo itself, with no enclosing membrane as such. Coal is crushed and compacted into a capsule shape. The capsule is then feed into a pipeline containing a flow of water. On arrival at its destination the coal log is crushed. The coal is then 'de-watered' by a mix of sedimentation and flocculation. CLP is still under development, although the first commercial installation is likely in the next few years. The most recent technical/mathematical summary of research into HCP is probably .
The use of linear induction and / or linear synchronous propulsion has been proposed as an alternative means of moving capsules within an air filled pipeline . An electromagnetic thrust would be induced in each capsule as it passed over magnetic induction coils set in the pipeline. In the case of linear induction propulsion these coils would be spaced at intervals within the pipeline. Linear synchronous propulsion would involve a continuous line of coils. As capsules pass through the pipeline at the same speed as one another, a constant volume of air remains between capsules. The proposed system would have a greater capacity than booster pump based PCP, and seems better able to attain high speeds (up to around 60 mph) and deal with uneven flow efficiently.
- Vance, L., Mattson, P., (1994), Tube Transportation, US Dept of Transportation, Volpe National Transportation Systems Center, Feb [summary cached text].
- Carstens, M., (1970), 'Analysis of a Low-speed Capsule-Transport Pipeline', Hydrotransport 1: Warwick, BHRA.
- Round, G. F., Marcu, M. I., (1987), 'Pneumocapsule Pipelines: Potential for North America', Journal of Pipelines, 6, 221-238, also presented to the Fifth International Symposium on Freight Pipelines, Philadelphia, Oct 13-16 1985.
- Knoll, E. G., Knolle (sic) Super Pipe, US Patent No. 4,024,947.
- Capsule Pipeline Research Center, http://www.missouri.edu/~cprc/, [cached text].
- Vandersteel, W., US Patent No. 4,458,602. A US Patent is also held by the University of Missouri relating to PCP systems based on linear induction motors.