1. If you can quote this letter, then you should be able to find the past discussion threads where it was made clear that SMRT/SBS has no statutory basis to stop anyone from photography.
2. By "statutory" I mean the ability to detain, fine or jail anyone for photography. SMRT/SBS are not police, they have no police powers. Hell, they are not civil servants or even public servants! As mentioned, they are employees of public listed companies.
3. Again, as has been discussed before, a company can eject a person (such as a photographer) from a place it owns simply because it owns that place, ie that place is not a "public" place.
4. Nevertheless, one constantly hears that our wonderful Govt pays for transport infrastructure such as bus interchanges and MRT stations, and that we the commuting public only pay the companies for the cost of buying and running their buses and trains. If this is true, then SMRT/SBS Transit does not "own" those stations and interchanges hence such places are "public" places where they can't eject you from.
PS Although we may use the term "regulations" very loosely (eg "rules" and "regulations" of a contest or game), "regulations" has a very precise meaning in law. It refers to subsidiary legislation which are issued by a competent Authority and backed by force of statute, meaning that non-compliance attracts criminal punishment, typically fine, jail or both.