Singapore simply has no space for more rails. For express or
rapid trains to service, there needs to be crossovers at stations.
Currently, the only feasible stations are Tanah Merah for the EWL,
Ang Mo Kio for the NSL, none for NEL and Paya Lebar for CCL due to
the extra middle track within the station. This is simply not
enough for express or rapid trains to function.
For more classes to be installed, at least 1 coach needs to be
sacrificed. This also means higher fares for refurbishing the
existing coach and lesser standing room for other normal class
passengers. On top of that, the coach cannot have walkthrough
functions. Plus the existing LTA rules of premium services,
passengers aren't allowed to stand in the premium coaches as well.
This will also cause platforms to drastically "shorten" due to the
normal train shortened to 5 cars or less. Japan can have it because
the Green Car takes up only about 1 or 2 cars out of the maybe
18-car train in the peak hours, with the platform much longer than
the train itself.
Unless the North Shore Line track layout hasn't been planned,
it's the only possible line to have extra crossover lines in each
station. But chances ar, it will not be carried out since it doen
not even lead to the CBD.
I hope the CCL use the same track system as DTL. Then trains can
be interchangeable. Like the EWL and NSL using the same track
system.