of a solid wall with heavyweight doorways and solid steel sliding doors, similar to a bank of elevators in a large building, and the train cannot be seen entering from the platform; passengers become familiar with the sound alone to indicate a train arrival. In May 2018, two other similar stations were opened: Novokrestovskaya and Begovaya. Unlike the first ten stations that were built, these stations utilize glass screen doors, allowing the train to be seen from entering the platform, like most other systems. It is unclear why platform doors were installed here as they are absent in all other metros in Russia, the CIS (except that of Minsk, shown above), or the former Eastern bloc (excluding Sofia, also shown above, albeit on a line with equipment incompatible with that of the typical Eastern bloc metro). The only other platform doors in Russia are found on the Sheremetyevo International Airport people mover. Singapore The Mass Rapid Transit (MRT) was the first rapid transit system in Asia to incorporate platform screen doors in its stations in 1987. Full height PSDs mainly manufactured by Westinghouse are installed at all underground MRT and sub-surface stations, while half-height platform screen doors were retrofitted into all elevated stations by March 2012. The LRT stations at Bukit Panjang, Sengkang and Punggol lack physical doors, only barriers with openings where the doors go (excluding the now-closed Ten Mile Junction station, which had full height doors) and vary in size according to their location on the platform. There are two variants of the full-height platform screen doors in use. The first variant, made by Westinghouse, was installed at all underground stations along the North South line and the East West line from 1987 to the completion of the initial system in 1990. The second variant incorporating more glass on the doors has since been used on all lines thereafter. Considered a novelty at the time of its installation, platform screen doors were introduced primarily to minimise hefty air-conditioning costs, especially since elevated stations are not air-conditioned and are much more economical to run in comparison. The safety aspects of these doors became more important in light of high-profile incidents where individuals were injured or killed by oncoming trains. In 2008, authorities began the process of retrofitting existing elevated stations with half-height screen doors. However, Land Transport Authority stated that the retrofit was not motivated by the need to