Have any buses been stopped by traffic
police for speeding?
No. In fact it is impossible for most buses to
speed here in Singapore as most are fitted with a 60kmph speed
limiter. Now you know why buses are slow even on expressways. Other
than DDs & Bendies which have been exempted due to the fear
that they may not be able to climb slopes easily (that's wat i
heard?), there are a few buses that are not fitted with the speed
limiter, and these buses are usually specially deployed on
long-distance express svcs (predominantly 858 ) to give it an extra
edge over other svcs. whether tis is illegal or not i dunno, but i
have sat on one 858 that goes at 120kmph for last bus, and surely
there will be others here that would testify that with their own
neck-breaking rides on some 858 buses. once in a lifetime
experience. 
In the olden days, it is possible they may get caught by the speed
cameras (especially Yishun Ave 2 & Holland Rd ones) but now
they can't even get caught if the drivers step hard on their
pedals.
How to know if a bus is fitted with a speed limiter: look out for
the Mannesmann Siemens VDO label near the front door/window
together with the road tax labels there.
Can I take pictures at
the busbay of 170/causewaylink at woodlands checkpoint?Are there
any sercurity issues?
You can do so, provided you do it fast and act natural, like out of
interest. Just don't get spotted by the staff, policemen and
possibly, the SBST traffic inspectors.
Why are there no new services by CSS
PTC wants to phase out CSS and thus no new services will be
introduced or amendments made to extend its current reach. In fact,
the CSS services are slowly disappearing year by year. 603 was the
last bus to go. However, they are allowed to keep running the
current routes, possibly cos they are still in (some) demand and
they don't wan to do "rationalisation " just for the withdrawl of
CSS services.
a short bit on the history of CSS/Tibs
Since the 1970s, CSS minibuses were running around providing
shuttle services between various estates & the CBD. In 1982 (or
around there), the government wanted a new competitor in the local
bus services industry to provide some competition and CSS was
chosen to break SBS's monopoly. In 1983, Tibs inaugrated with 2
services (164 & 167 i think), and both CSS & Tibs became
subsidiaries under Tibs Holdings.