When all your enemies are beaten and a Division of Armor units siege the last city of your last opponent, time is right to develop your civilization. After the invention of railroads you should starve cities you do not need anymore away to optimize the map. Make sure that before building a nee city you build railroads on the spot to get the full possible bonus. You cannot railroad a square where there is already a City on it. One extra food may be relevant to have a population of 39 or 40, Meaning 400000 inhabitants more or less. Only by railroading your ancient cities. But take care you do not starve away your world wonders. Even better to build wonders in only one City which is perfectly placed on the map and is situatet in a Forest/plains area for higher Production.