Pop almost definitely will hit 6m within 5 years. 107K were
added in 2011 alone, we should stand around 5.25 now. As more PR
become citizens, more prospective PR become eligible due to family
ties, so the rate of immigration will increase.
As for the North Shore Line, it won't be so fast. First the
areas all around the Circle Line, and around the DownTown Line in
both Bt Timah and Thomson will be fully developed.
Subsequently the North Shore Line planning will get more
serious. Then the feasibility study. Then construction in stages.
My guess is you can expect a North Shore Line around 2030.
Singapore infrastructure always lags behind, not leads, population
growth.
So we can expect mrt to breakdown once every 2
days? 
6 million is no joke. 