Aiyah, I just tell you all the answer, I got owned by a rebuttal
point, and I'm not sure how to explain it, LOL!
NEL was originally planned to be joined to Malaysia, and then
underground of the North of Punggol, the tunnel is already built,
and there's a gate (or something) in the tunnel further north of
the station to block trains and people from continuing deeper into
the tunnel.
What I'm not sure is how deep is the tunnel going through after
that gate.
Hmm interestingly. Certainly I did not know.
Look, a source on LTA 1996 white paper showed that on Page
58/76

And that entire document can be hard to find (Except NLB), and I
once got it, saved it and re-upload to my 4shared:
http://www.4shared.com/document/X_TDbS35/LTA_white_paper.html
Well, indeed Singapore is looking on possibility to extend MRT
network to Malaysia and it supports Flip's statement. But, the plan
did not work, most likely due to political than economical reasons
(One West, One North checkpoint, certainly it make sense to have a
checkpoint in the East).
Any case, facts is as above.