I don't see a reason why more MRT stations will remove the need
of DDs and bendies on feeders. In fact, even more high
capacity buses will be needed to cater to the demand from the new
MRT stations to neighbourhoods.
Simply, you cannot put an MRT station every 400 meters. And you
cannot expect everyone to walk from the MRT stations to their home
(some 500m, some 1km, some 2km etc). The fact that even short
feeders like 225 and 284 survive shows how willing people are
to walk all the way back when there's a convenient feeder. Are you
really gonna build an MRT station at Bedok North St 3 to
replace 225, which adds 2-3 mins of travelling time for other
commuters? Don't expect LRTs since LTA has stopped planning
any LRT lines for a long time.
And I can tell you, the rate at which LTA is planning the MRT
lines is really slow... TEL Thomson part only ready by 2021, East
Coast part by 2025, CRL by 2030... And even when these lines can
significantly reduce the need of some trunk services, they cannot
do so for feeders. Even with these lines, the density of MRT
stations is still not enough to have one station in every
neighbourhood etc. In most of the towns, buses are still the way to
go.
If you really want everyone to take MRT, and wish all those who
are taking direct trunk/express/premium services or
even driving cars to switch to feeders to transfer to
MRT, How can only SDs (rigids) cope with this demand?
For what they plan fast? Wouldn't that means that they have no
job? Hahas..unless they got more space to dig..I think S'pore can
prepare to introduce Double Decker trains for NSL/EWL to cater to
the needs
From what I know, S'pore will have DTL, TEL, NSL, EWL, NEL, CCL
only until 2030.
Overall, it's seriously poor planning for the transportation
system.."Micro-sized" interchange with "Mega-populated" crowds..BIG
dreams indeed..
Anyway, if you think the operators alone going to bring in more
double decks, wait longer maybe? It's gonna be more SDs. SD(lower
run cost) + pack bus(more profits) and with more drivers(-ve
part).. Because during the off peak, your DD is literally
useless.
(Getting more DDs?)Good luck climbing up and down
people 
