The H-Bahn (abbreviation for , German for 'hanging railway') in Dortmund and Düsseldorf (there known as "Sky train") is a driverless passenger suspension railway system. The system was developed by Siemens, who call the project SIPEM (SIemens PEople Mover). Two installations exist, one at the Dortmund university campus, the other at the Düsseldorf Airport. While Siemens is no longer actively marketing the system, and will no longer carry out turnkey projects, new installations are still possible in collaboration with the Dortmund operating company. Since 2011 Air Train International has been marketing the system in China and as of May 2013 there are proposals to build lines in Shanghai and Wenzhou. A number of other Chinese cities are also studying the system. Technical description The cabins are centrally controlled and do not need a driver. The system can operate on a schedule or on-demand, whereby a passenger requests a carriage by pushing a button, similar to summoning an elevator. The maximum speed is 50 km/h. The system allows forking by a system of switches in the carrier. Suspension and propulsion The carrier is a hollow rectangular box girder with a slit in the bottom through which the cabin is suspended at the running gear, whose two axles carry the load with a rubber wheel on both sides providing both suspension and propulsion. Two wheels run horizontally along the top and bottom of the interior side walls of the carrier box, providing horizontal guidance. Thus, the designation as a monorail system is to be taken with a grain of salt, meaning just that there is a single axis of suspension. 400-volt three-phase current is taken from four conductors at the side wall. Above those, a cable provides continuous wireless data connection between the train and the control center. All contact between the suspended cabin and the fixed system is enclosed in the interior of the carrier box, protected from the elements. In this sense, the SIPEM suspension system is similar to the one used by the earlier SAFEGE system (which was developed in France and which is used on two networks in Japan), but is much narrower, both the carrier box as well as the open slit for the suspension. The cabin is suspended below two motorized bogies, which are enclosed in the carrier girder. Each bogie is equipped with two 31 kW electric motors, providing a peak motive power of 124