Taking the food (in this case) to your own city would be fine as it is for your own good. But as said before, there's no clear line between stealing for competition and down right sabotaging.
I only have experience playing on #7. There it's obvious that if you try to "steal" pax from someone else's city, you also limit the profit of those trains and trucks you use to do so. And since you only got 10 trains 20 trucks it really hurts yourself unless if it's still a decent connection.
So on city builder games:
- if you can use the recourse (food, goods,...) to help your own city grow and thus helps you in getting your goal. By all means, try taking it from your opponents as it is a win-win for you.
- If you just take the recourse to make a profit from it, it's vague, I'd say it is fine if a neutral player (someone not involved in either company) can say that it looks like it will make a decent profit.
- If it means slowing down the other player just enough so YOU can WIN yourself, I'd say it is allowed but should be avoided in most cases as it will most likely upset the other player (unless if you are having fun in chat and are competing in such a way so both parties like this kind of competition)
Note, in that last case you should both be competing for first place with not too much difference. for example both around 90% or both around 50% (with other players 40% or less).
If the other player really is faster/better, try figuring out what he did different. And learn from it. You can even ask them for some tips. In my experience everyone that isn't speed-building at that moment wouldn't mind giving you a pointer or two on how to improve your game.
These servers are here to have a fun competition between players. In my opinion it's way more fun to play against players who are equally good. So if I'm not the best, I try to learn from the better players. If I am the best, I try to educate other players so I get my fair competition going. Win-win for everyone