I've completely given up assigning trade routes ... just given up. Here's what I do instead:
I set all resources under the Governor to "export." Lumber, Tools, Weapons and Horses I also set to "Import" and set each to my starting warehouse capacity (300 at the settings I play at). My port closest to Europe I set to import all finished goods, and cities I make into goods producers I set to import the raw materials I'm going to work in them, of course.
I like huge maps and Marathon games, but the trade routes just get unmanageable quickly. By using fully automated Wagon Trains, I don't have to worry so much about it. Its not quite as efficient, and occasionally you run into snafus (like your Wagon trains getting clogged with lumber faster than your cities can use it if you aren't being careful) but overall, I found it to be much less of a headache than the pain-in-the-tail trade route "management."
I set all resources under the Governor to "export." Lumber, Tools, Weapons and Horses I also set to "Import" and set each to my starting warehouse capacity (300 at the settings I play at). My port closest to Europe I set to import all finished goods, and cities I make into goods producers I set to import the raw materials I'm going to work in them, of course.
I like huge maps and Marathon games, but the trade routes just get unmanageable quickly. By using fully automated Wagon Trains, I don't have to worry so much about it. Its not quite as efficient, and occasionally you run into snafus (like your Wagon trains getting clogged with lumber faster than your cities can use it if you aren't being careful) but overall, I found it to be much less of a headache than the pain-in-the-tail trade route "management."