airports

[ramses II.]

I often found cities on islands far, far away from my empire only for trade use: In the later game, I build airports and highways in that city. Then I send freights through the air to that city ... CA$H.
Three or four of those far-away-cities will give you a constant stream of gold (and beakers!). In one of my games, I earned more than 2.600 gold for only one freight of oil.

[Marquis de Sodaq]

Yes, an excellent tactic in the late game. The complete trade payoff formula was found by William Keenan. Somewhere in this forum is his thread about it, aptly titled "Caravan Delivery Payoff Formula" or something like that. Handy info to have!

In the early game, just delivering to another island can give an equally (% anyway, not total gold/beakers) big boost.

[markusf]

unfortunately islands are useless for trade. They give a big trade bonus but the trade routes will always be 50% of a trade route that is on the same landmass as you. Islands only give full trade amount if the 2 islands are more then 15 or 20 squaures apart (don't remember the exact number)

[Thoddy]

The distance between trading cities must not higher than 23 squares. If its so, road and railroadbonuses are lost. Mean
1/3 bonus if cities left by road
2/3 bonus if cities left by railroad
remark a 50% bonus is incorrect as described in the scrolls of ancient wisdom.

This means, only the 2 squares located at the optimal path within cityradius must have a road or railroad improvement.

To check out where the optimal path is situated you can use the Explorer unit (road flag) with an go to order)

The road/railroad bonus is also lost if there is a enemy unit sitting on the optimal path within cityradius or trading over the zero meridian, even the cities are closer than 23 squares.