That's a good question. I don't really know, seems rather irrelevant in this iteration of the game. My experience is if the trade view is cluttered it is because you either didn't start routes, they're out of range, or you hit the trade limit with that civilization. You start with one per civilization and it increases by one each time you improve trade relations. If you're out of range find a way to increase trade routes distance, like convert to a trade town, or build something closer. Once you establish a route it disappears from the list. There's no hard limit to trade routes. You have to actually send a trade unit out to the city you're establishing a route with but you don't select which of your cities it's with. I think it's just whatever is closest. Once it's in your trade network it doesn't matter what city.
I played for a while before I realized that what you see when you click on the city banner is just the build queue. There's a whole separate city info screen. There's a button on the build queue to open it and it has like four buttons to view various information about the city. You might check there to see if that lists the trade routes to and from the city. If not check mods like City Hall to see if they added it. I think mostly people view it as irrelevant due to how they work now. They're more concerned with routes between towns and cities. Since the roads are bit ambiguous in showing what's connected several mods show the connects to the cities.
