they limit the kind of rolling stock that may be used on a line, as train doors must have exactly the same spacing as the platform doors; this results in additional costs due to depot upgrades and otherwise unnecessary purchases of rolling stock. Despite delivering an overwhelming improvement to passenger safety at the platform-train interface, platform screen doors do introduce new hazards which must be carefully managed in design and delivery. The principal hazard is entrapment between closed platform doors and the train carriage which, if undetected, can lead to fatality when the train begins to move (see ). Cases of this happening are rare, and the risk can be minimised with careful design, in particular by interlocking the door system with the signalling system, and by minimising the gap between the closed platform doors and the train body. In some cases active monitoring systems are used to monitor this gap. Half-height platform edge doors, also known as automatic platform gates, are cheaper to install than full-height platform screen doors, which require more metallic framework for support. Some railway operators may therefore prefer such an option to improve safety at railway platforms and, at the same time, keep costs low and non-air-conditioned platforms naturally ventilated. However, these gates are less effective than full platform screen doors in preventing people from intentionally jumping onto the tracks. These gates were in practical use by the Hong Kong MTR on the Disneyland Resort line for the open-air station designs. Most half-height platform edge door designs have taller designs than the ones installed on the Disneyland Resort line. Rope-type screen doors There are also rope-type platform screen doors, where multiple train types with different length and train door structures use the same platforms. The barriers move upwards, rather than sideways, when letting passengers through. Some Japanese, Korean, Chinese and eastern European countries have railway stations that use rope-type screen doors to lower the cost of installation and to deal with the problem of different train types and distance between the doors. Use Argentina Line D of the Buenos Aires Subte is planned to have platform screen doors installed in the future as soon as the CBTC system is installed. Australia Sydney Metro Northwest, the initial section of the Sydney Metro, which opened in May 2019, was the first-fully automated rapid transit rail system in Australia. Full-height screen doors are provided on most underground platforms,