Internal Trade Routes - Food and Production by Era
Sea Trade Routes carry double, just as they do in Gold Trade Routes. The amount of Food/Production sent through a Trade Route is determined by the current Era. It is very handy to send your Capital Food from other Cities to allow it to work Specialists, like Writers, Artists, and Musicians, without harming Growth. There is a Steam Achievement for sending 3 Caravans to a City carrying Production to help speed construction of a Wonder. You should almost always use some internal Trade, for they can benefit you greatly. Food in particular will speed the growth of Cities, resulting in more of everything - Science, Gold from Tiles/City Connection, and Production. You get the following yields:
Ancient Era - +3 Food/Production from Carvans, +6 Food/Production from Cargo Ships
Classical Era - +4 for Land, +8 from Sea
Industrial Era - +5 for Land, +10 from Sea
Modern and Later Eras - +6 for Land, +12 from Sea
Sending Food and Production from a City does not subtract from its yields. This is simply a bonus for trading between Cities, and nothing seems to impact yields but the Era. The only means of boosting this otherwise is the Iron Curtain Order Policy to raise yields by 50%.