While there are rules in trains that eat and drinking can merit
a $1000 fine, we should not get carried away entirely simply
because a person falls in that catagory.
The crux of not eating in MRTs is basically to prevent:
1. the proliferation of pungent food smell
2. dirtying up the localised area
3. basic courtesy to your fellow commuter
If a person can do eat/drink WITHOUT violating the above 3, then i
have no issues. Usually, examples are taking medicine, drinking
plain water from a carrier bottle, baby drinking from a milk
bottle, popping a sweet into your mouth etc.
Anything along that line should be fine.
Eating breakfast on the train (any form of breakfast for that
matter) should not be practiced and should be treated as a
violation. (inset: prompting for citizen's duty to tell the errant
guy off).
Worst kind is parents giving food to their kids on the train. These
families should be THROWN OUT by their fellow commuters. 
