Yep. The highest food tile is the floodplain wheat. Flooplain is 3 food, + 2 food for wheat, + 2 food for irrigation and railroad = 7 food per tile. A city can work 20 tiles plus the center square, so if every tile in the city's radius is an irrigated railroaded floodplain wheat, it makes 140 food per turn + 3 from the city square (if you're agricultural). Every citizen requires 2 food per turn, so the city can have 71 citizens and be growing at one food per turn, or have 72 citizens and be starving at one food per turn.