Trade Routes in Endless Space are automatically formed when peace is made with another empire, and at least one of their systems has been explored. Every destination system in a friendly empire can form one trade route with one of your systems. Trade routes are formed and change automatically as new destinations are found.
It should be noted that trade routes are not mutual. Each empire calculates its own trade routes separately, and trade is 'one way', like exports or deliveries. Destination systems don't need to 'accept' the trade route, nor is their capacity used up if they are part of your trade routes.
The home world system by default has two trade routes, and as such is capable of creating trade routes with two other peaceful systems. In order to attain even more trade routes, system improvements must be built to increase the maximum trade routes a system can have. Trade routes however can only link up to one system, so in order to put trade routes to good use, your empire must have explored as many peaceful systems as trade routes.
Establishing Trade Routes between systems generates income in the form of Dust () and Science (). The income generated by a Trade Route depends on the population, system improvements and any potential luxury resources. Trade routes are automatically established to be the most profitable for the player.
Trade routes provide both dust and science, making them a good source of income. The amount of dust and science is related to several properties: population, distance, improvements and heroes. The more population a system has, the more dust and science a trade routes produces. Therefore, prioritize high population systems with trade route improvements.
Distance also comes into play. The farther the distance to the system, the more dust and science the trade route will produce. Improvements will increase the maximum number of trade routes, or give bonuses to existing trade routes. Heroes that specialize as corporates are able to gain abilites that increase trade route income and maximum number of trade routes.
It should be noted that trade routes are one way (i.e., they are not bidirectional). Trade routes are established with peaceful systems, regardless of those system's number of trade routes, improvements or population.
Blockade Breakers & Cooperation Agreements
By default, trade routes are only established when you are at peace with another empire. With Blockade Breaker, a faction trait, it is possible to establish trade routes while at Cold War, or even at War. This means that upon discovery of another colonized system, be it peaceful, neutral, or hostile, your empire will gain another trade route.
Cooperation Agreement is a diplomatic action that must first be researched. Once researched, your empire can offer a cooperation agreement to another empire. If it is accepted, for the first 15 turns all trade routes in both empires will take a penalty. For the next 15 turns, trade routes will return to normal, and the 15 turns after that all trade routes in both empires receive a massive bonus to the amount of dust and science produced.