"Trade is not handled via Civ-style Caravans. Trade is now handled as a function of your diplomatic interactions. If you are at peace with someone, you are assumed to have a certain level of trade with them. The closer you are as allies, the more trade you have between your two countries. This adds an interesting angle to your decision to be at war with someone, because it sacrifices your trade. Other countries may also impose sanctions on you for the use of Atrocities, which then reduces your trade."
Tim Train of Firaxis. From Dante’s Inferno SMAC page. Link to source. SMAC-FAQ