you can eat on public transport, but what you can eat depends on the mode:
bus? only snacks basically, maybe a small sandwich
tram? mostly the same, but you can probably get away with a bag of mcdonalds if you're low-key about it
metro? i'd be quite comfortable with a bag of mcdonald's, a hand-held kebab would probably fly but not anything you need utensils to eat
and lastly on a train i'd say you can eat anything you'd eat at work, so basically just not a whole pizza