by Duke o' York:
...The size of the bonus - in gold and science beakers - depends on a number of factors: whether the target city demands the commodity in the caravan, the distance between the cities, whether they are on different islands, which civilisations they belong to and the current state of relations between civilisations, which improvements they have in the cities (marketplaces, banks, stock exchanges, superhighways and ariports all improve the pay-off) and whether the cities are linked by road or rail...
I might add a few clarifications...
Trade is a complex subject (if treated in detail like my 10,000 char posts last year), but in fact the bonus does not depend on the "current state of relations between civs"... you probably meant the indirect influnce of the types of Government. But the game actually uses the number of uncorrupted natural trade arrows (but not the arrows due to trade routes) in each city as the basis. Govt's affect the amount of trade arrows, but do not enter into the trade bonus or the recurring trade route value.
Also, the market, Bank, and Stock Exch have no effect at all on trade... they merely amplify the allocated trade arrows. However, in Despotism, Monarchy, and Republic, the Courthouse can drastically affect trade (cuts corruption by 50%). The SH will grant a 50% bonus, plus add 50% more trade arrows on tiles which generate arrows (rounded down). The Airport will only generate its 50% bonus if and when both the source and destination have Apts.
About roads & RR's.... each generate 50% extra.... but with a major caveat... the road/RR must take the exact route the AI "expects".... if even one link is broken (or the link is complete and any enemy is located on an enroute link), then the bonus is removed (the details of this warrant a whole separate discussion). Big note: Airfields do not count as either a road or RR link.
Further, if the destination city is yours, you lose 50%.
Another bonus is the Freight (50%) vs caravan.
If cities are on separate continents, you get a 100% bonus (the same as Road + RR).
Also, there are bonuses based on techs discovered... which is the reason I delayed my GOTM 16 discovery of navigation, BTW. If the game is not yet past turn 200, and both Nav and Inv are not discovered, the trade bonus is doubled. After RR, it is cut by one third. After flight, it is cut by another third (leaving one third of the "normal" bonus!!). These are significant milestones.
Well, I won't go into more detail for now, but I think a search using starlifter and trade and maybe democracy or power democracy (time period of last year) would pull up lots of stuff if someone wants more.
The general thrust is that trade, while not essential to winning Civ 2, will provide the player with faster growing cities (due to lots of happiness in Republic/Democracy), lots of Gold, and starting about Corporation, one advance per day with 0%science (due to steady flow of delivered freight and an Einstein), and of course lots and lots of gold. It can become so powerful that I almost feel sorry for those binary bits and bytes trying to outwit the player

.....