Aha... found this page: http://en.wikipedia.org/wiki/Braddell_MRT_Station
"Braddell MRT Station (NS1
is located on the North South Line on the Singapore MRT.
The station is an underground station and is located in the nothern
part of Toa Payoh. This is also the only underground station which
have side platform.
When the MRT system was built in the 1980s, SMRT tried to design
the track system and position in order to enable the station to
have a common platform, like in the mainstream MRT stations.
However, the closly built HDB Blocks in Toa Payoh estate meant that
the MRT system tunnels had to be constructed directly under Lorong
2 Toa Payoh, following the bend of the road, in the area between
Lorong 1 Toa Payoh and Toa Payoh MRT Station. The result made it
impossible for tracks to diverge at a common platform station, as
the angle Southbound trains would turn when approaching and leaving
the station would be too great.
It would be unfeasable to change the position of the station,
because moving it south would render it too close to Toa Payoh MRT
station, while moving it north would be impossible because of
surrounding HDB blocks and JTC Flatted factories. This position
would be the best for the station. Therefore SMRT had no choice but
to build the station with a side platform."
