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Why are they double decker trains?

Aeroexpress Kiss EMUs enter revenue service
http://www.railwaygazette.com/news/passenger/single-view/view/aeroexpress-kiss-emus-enter-revenue-service.html

Considering the capacity of Aeroexpress, I wonder why the trains have to be double decker trains.
Now, Aeroexpress train has been operated at the interval of 30 mins. So, it's not difficult to operate trains more frequently, instead of installing double decks.