You need the trading posts in each intermediate gravity well to get the trade route length bonus. Any gravity wells in between them that have logistics slot capability but DON'T have a trade station break the chain and you don't get the longest-path bonus.

Note that a "dead asteroid" counts as an interruption, and they can be a real pain in the ass to a trader economy. If your chain of planets looks like this: terran-asteroid-ice-volcanic-desert, your chain is five. But if it's like this: terran-deadasteroid-ice-volcanic-desert, your longest chain will only be three.

-- Retro

How does a trade agreement with another player affect trade income?

Here's a question I have about how the game calculates which of your routes is the longest. This happened to me in a game, I believe on the Centrifuge small map.

Please forgive my crude diagram. The I's and the dashes are phase lanes. The dots are just there to get the diagram to line up properly, just blank space.

A--------AF
I...............I
I...............I
I...............I
TP--------DP

I had a trade port constructed at the Asteroid at the top left of the diagram, the Terran Planet at the bottom left of the diagram, and the Desert Planet at the bottom right. All of these trade ports were trading with each other. However, the game decides that my longest trade route is from the Asteroid at the top left to the Desert Planet at the bottom right, but instead of going through the Terran Planet it goes through the unoccupied and uncontrollable Asteroid Field denying my my trade bonus on the Terran Planet. The distance from the planets looks to be equidistant, but the Terran Planet is completely ignored for the purposes of the chain. Why does it do this?

Mmmmh... that informative.