if u wanna strictly abide by their *rules*, it's there in fine print outside the ticket gates. the MRT is not public property, and they do have rules for (against) photography, though they may not always enforce them.
If you are asking, cannot. If official photo shoot, it's a different story, but if you just happen to be taking photo of your friend (who is ok with it), and the background is part of the mrt stn, why not? Just don't be giving the impression that you have something to hide.
If you take a few quick shots they won't bother. They know that there's a pretty large grp of photo enthusiasts who're doing it. But if you linger around for hours snapping at everything they're bound to be suspicious.
Ah..you mean the riot police that do periodical visits at the station? heh Well you ask them lah. See if they willing to pose for a few shots with you or your friend. Tell them you local and gladly let them take down your IC. That you like to take their picture as you are proub of what they do. heh. In my days in the riot police when we patrol places, we get photographed by folks who come up to ask us and sometime we don't like it not because of sercurity but scare we might be caught not standing correctly, Uniform not properly worn, not wear our baret or peak cap while on patrol...etc heheh or worst caught disturbing girls! Usually NS boys that does this bit lah... heheh. But these day the riot police are no longer NS men but regular officers (Regulars might not be happy you take their pictures for REAL sercurity reasons heh ) unless they are from the KINs units. They are police reservist who are deem fit enough ( below 30yrs) to join this special tactical group. very cool... wish I was still young man..
Shot them doing their patro might be a problem lah but if say your mother or girlfriend want to pose a shot with them..that is okay heheh.. It is good Police public relations too.