By the time I moved from Hong Kong in 1992, the relatively small MTR network had a daily ridership of over 2 million and was beginning to reach its original fleet’s ridership capacity. The original trains were delivered from 1979 to 1992 and look like this:
The MTR’s first fleet enhancement was ordered from KOROS (now Hyundai Rotem) of South Korea, which resulted in a design that carried the original design theme with a modern twist. Propulsion was Mitsubishi IGBT-VVVF, and trains first began operation in 2002. The first major difference is the gloss grey paint replacing the flat white (the striping is for a special line with higher speeds):
The most unique feature to me, though, was its use of novel plug-style doors for wind noise control. Here they are in operation:
Many types of rolling stock in Europe use doors that sink in via curved rails and springs (closing with a loud, uncontrolled impact), but these doors are pulled in and out with what appear to be electric motors yielding a slow, controlled in/out motion.
Not sure why this complexity was desired, but subsequent trains from other suppliers have reverted to conventional sliding doors located outside of the train body, and offering the same quietness advantage over the original trains.