NS6 was supposed to be part of the Woodlands MRT extension in
the initial plan. The station was due to be located in the Sungei
Kadut Industrial Estate. However, it just did not make sense to
have an MRT station in the middle of an industrial estate, and the
plan for the station was scrapped. However, the alphanumeric code
has been left empty as there may be 'future developments' to
'warrant cause for building and operating the station'.
NS12 was due to be built in between Yishun & Sembawang
stations, where a new housing estate Simpang was proposed. However,
due to a change in plan, the Simpang HDB estate project was
indefinitely shelved. NS12 is a provisional station which will be
built if URA sees the need to redevelop a completely new housing
estate there, though there are currently no plans to do so, at
least not in the near future.