Roland Johansen
Deity
Foreign trade routes are 2.5 times as profitable as similar domestic trade routes, so you would want to get as many of them as possible.
To get foreign trade routes, you need to have open borders treaties with civilizations with big cities and know a path to their civilization. You also need to be able to trade using a path that leads to their civilization (you can't use the ocean to trade until you've discovered astronomy). If the cities of the other civilization are distant from yours and large and your cities are large also, then trade routes will be even better. A coastal city with a harbor will get another 50% bonus on trade income.
So you need to do some exploring (or buying maps later in the game) and you need some civilizations who like you enough to get open borders.
Each foreign city can get a single trade route with one and only one of your cities, so in the late game when you get multiple trade routes per city, you would like to have lots of civilizations with open borders to get lots of foreign trade routes. When you don't know enough foreign cities to trade with, then the remaining trade routes will be domestic ones and thus far less profitable.
Oh, I forgot one thing: the mercantalism civic stops foreign trade routes whether you use it or your potential trading partner uses it.
