Back to Trade route
The Trading Post is a special building in Civilization 6.
Game info
Whenever a civilization finishes (that is, the route runs its alotted number of turns) a trade route to a certain city for the first time, it creates a Trading post in the city center district of the destination city. Trading posts can exist in both foreign and domestic cities, and multiple civilizations can establish trading posts in the same city. Civilizations cannot make use of trading posts established by other civilizations.
Trading posts extend the range of trade routes which pass through their cities by 15 tiles, which is also the base trade route range. If a trade route reaches a city with a trading post, it may then continue up to fifteen additional tiles to reach another city. If that city also has a trading post, the route may extend a further fifteen tiles, and so on. Each trading post also adds +1 
Gold to every route originating in the trader civilization which passes through its city.
Strategy
Trading posts in foreign cities are vital for extending trade range far beyond a civilization's borders. Domestic trading posts can allow a particularly productive core city to supply a developing border city directly, and can also enable wealthy core cities to trade directly with foreign cities.
The Roman civilization ability All Roads Lead to Rome gives a further +1 Gold to Roman trade routes which pass through a Roman city with a trading post. It also provides a free trading post to every city Rome founds or captures, as well as a road if the city is within trade range of the capital. This ensures all Roman trade routes, especially those originating in the capital, have long ranges and a wide choice of destination cities.