Trade routes require a “Home” which is a building with the Trade Site tag (Market, Dock or some landmarks) and a “Target” which is a building with the Trade Site tag owned by either an ally, enemy or neutral. Using a neutral market gives a 20% increase in collection. Traders will automatically find new Homes, and don’t care about which type of building is used, so if your market is destroyed (or relocated for Mongols) your traders might automatically swap to a dock.
Income appears to be quadratic with length, so longer trade routes are strictly better, though obviously harder to control safely. Trade routes that are so short they display as +0G on the market do eventually provide income, so fractional amounts are counted. This also means the exact quadratic below is not correct, because it should be positive at zero tiles edge-to-edge.
Income is map size dependent, a 90 length route provides 173 per trip on micro but only 112 on large. On the other hand, a 170 length route on large provides 373 per trip, so better routes are possible on larger maps.
When commanding traders, they have two commands called “Trade” and “Set Home Market” that allow you to change the route. Right clicking a Home Market works just like “Set Home Market”: return leg traders auto update their current cargo to the new amount while empty carts continue towards the Target while remembering the new Home. Right clicking a Target is identical to the “Trade” command: return leg traders will drop cargo and go there immediately (even if it’s the same Target, which is not ideal), while empty carts go towards the new Target immediately. Shift-Right click is extremely dodgy and can result in no change to the route or idle traders, so I would strongly discourage its use.
So to be clear, if a trader is on the return leg with resources and the Target is changed, he loses his carried goods and starts the new route. On the other hand, if the Home is changed, his carried goods automatically updates to the new value and he continues to drop off without needing to visit the Target again. Unlike AoE2, there is no “dud trip” where he uses the old amount for a longer route. This can be used to your benefit when mass-producing a trade route: Build your ideal Home far away from your target, then more markets near the Target market. The additional markets can be rallied at your Home then shift queued to the Target, and produced traders will immediately start the route at the Target and skip the first trip.
Sea routes are vastly more pop space efficient. If a land route produces X gold, a sea route of the same length provides 2X gold and X wood, and trade boats move at 1.5 Tiles/second and upgrade to 1.725 while land traders are 1 T/s and don’t have an upgrade. They are more expensive and take longer to build, and water is of course harder to fortify, and there’s French out there.
Trade ships do seem to bump a lot, especially if their route hugs a coastline.
Now a couple of Mongol specific points. The Silk route counts the sum total of both land and sea traders, they don’t need to be on the same route or share either Home or Target.
Traders will update their carried gold while a packed market is moving, but can’t drop off until the market unpacks, so this is almost certainly trivia.
The Yam speed bonus from outposts applies to both land and sea traders even before the Castle Age/Deer Stones upgrade. Units also retain this buff for 20s after leaving the area, so you don’t need to coat the route in outposts (although more vision and garrison space is always valuable). The Khan’s Maneuver Arrow also applies to traders, so I think for the next 5s this Yam/Sail/Arrow sea trader is the fastest economic unit in the game:
Lastly, when in Observer/Replay mode, it appear the coastal trade posts are owned by Rus/English hybrids with vanguard man-at-arms, early knights, spearmen and archers. So you better watch out if they ever they ever swap diplo and fight back. Land trade posts on the other hand are completely harmless.