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Cyrion - Sorry, I should have given a more complete answer to Elephant's question about Base Trade. Here it is:
Base Trade
The basis of all trade is terrain and its utilization by cities. By working the resources of squares in their radius, cities generate trade arrows which can be allocated to taxes, research, and luxuries. There are four sources of trade arrows from terrain:: oceans, rivers, roads, and trade specials.
Ocean squares generate 2 trade arrows, the whale special generates 3.
Rivers add 1 trade arrow to any square they flow through, regardless of whether that square is capable of producing trade on its own. Although any terrain type except ocean may have a river in it, the most commonly encountered rivered squares are grassland, plains, forest, jungle and swamp.
Roads have several trade related affects. A road through grassland, plains or desert will enable the square to produce 1 trade arrow. Since these terrain types support no trade-producing specials, roads are the only way (except for rivers) of generating trade from this terrain.
A second affect of roads is that they increase the trade of any square which is already producing trade by 1 arrow. For example, a rivered forest square produces 1 arrow, building a road there (technically, a bridge) increases the trade to 2 arrows even though a road through forest terrain normally will not generate any trade. Roads do the same thing for trade specials, they increase the trade produced by 1 arrow. Railroads have the same effect as roads.
Trade Specials are, perhaps, the most important source of trade and cities are often founded nearby to take advantage of them. The trade specials and their terrain types are:
Fruit jungle 1 arrow
Silk forest 3 arrows
Furs tundra 3 arrows
Ivory glacier 4 arrows
Wine hills 4 arrows
Gems jungle 4 arrows
Spice swamp 4 arrows
Gold mountains 6 arrows
There are three additional factors that can modify the base trade that a city produces from working its terrain: government, the Colossus wonder, and the Superhighways improvement.
Government can have an important affect on trade. The Republic and Democracy forms of government cause every city to produce 1 more trade arrow from each square that is already produces trade. Under Monarchy, Communism, and Fundamentalism this same effect can be obtained when cities are celebrating WeLoveThe****Day. Type of government also affects the amount of corruption which cities distant from the capital experience. Corruption causes a reduction in the Base Trade of a city.
Colossus works much the same as the Republic and Democracy government types, it adds 1 trade arrow to every square that is already producing some trade. The effect of the Colossus expires with the discovery of flight.
The Superhighways city improvement has a pronounced affect on all aspects of trade: base trade, trade routes, and delivery bonuses. It increases the trade produced by each worked city square with a road (or rail) by 50%. Since this calculation uses Civ's integer math, a square must have at least 2 trade arrows to be affected by Superhighways. Thus, a plains square with road and Republic will increase from 2 to 3 arrows. A rivered forest square with a road, Colossus, and Democracy will increase from 4 to 6 arrows. A Gold special with road and Republic will increase from 8 to 12 arrows. It is especially important to have roads built on all trade specials before the Superhighways improvement to gain maximum benefit from this improvement.
The total trade output from terrain, road improvements, government, the Colossus (if present) and Superhighways make up a city's Base Trade. The Base Trade of a city is an important determinant of the two other aspects of trade: trade routes and caravan delivery bonuses.
Base Trade
The basis of all trade is terrain and its utilization by cities. By working the resources of squares in their radius, cities generate trade arrows which can be allocated to taxes, research, and luxuries. There are four sources of trade arrows from terrain:: oceans, rivers, roads, and trade specials.
Ocean squares generate 2 trade arrows, the whale special generates 3.
Rivers add 1 trade arrow to any square they flow through, regardless of whether that square is capable of producing trade on its own. Although any terrain type except ocean may have a river in it, the most commonly encountered rivered squares are grassland, plains, forest, jungle and swamp.
Roads have several trade related affects. A road through grassland, plains or desert will enable the square to produce 1 trade arrow. Since these terrain types support no trade-producing specials, roads are the only way (except for rivers) of generating trade from this terrain.
A second affect of roads is that they increase the trade of any square which is already producing trade by 1 arrow. For example, a rivered forest square produces 1 arrow, building a road there (technically, a bridge) increases the trade to 2 arrows even though a road through forest terrain normally will not generate any trade. Roads do the same thing for trade specials, they increase the trade produced by 1 arrow. Railroads have the same effect as roads.
Trade Specials are, perhaps, the most important source of trade and cities are often founded nearby to take advantage of them. The trade specials and their terrain types are:
Fruit jungle 1 arrow
Silk forest 3 arrows
Furs tundra 3 arrows
Ivory glacier 4 arrows
Wine hills 4 arrows
Gems jungle 4 arrows
Spice swamp 4 arrows
Gold mountains 6 arrows
There are three additional factors that can modify the base trade that a city produces from working its terrain: government, the Colossus wonder, and the Superhighways improvement.
Government can have an important affect on trade. The Republic and Democracy forms of government cause every city to produce 1 more trade arrow from each square that is already produces trade. Under Monarchy, Communism, and Fundamentalism this same effect can be obtained when cities are celebrating WeLoveThe****Day. Type of government also affects the amount of corruption which cities distant from the capital experience. Corruption causes a reduction in the Base Trade of a city.
Colossus works much the same as the Republic and Democracy government types, it adds 1 trade arrow to every square that is already producing some trade. The effect of the Colossus expires with the discovery of flight.
The Superhighways city improvement has a pronounced affect on all aspects of trade: base trade, trade routes, and delivery bonuses. It increases the trade produced by each worked city square with a road (or rail) by 50%. Since this calculation uses Civ's integer math, a square must have at least 2 trade arrows to be affected by Superhighways. Thus, a plains square with road and Republic will increase from 2 to 3 arrows. A rivered forest square with a road, Colossus, and Democracy will increase from 4 to 6 arrows. A Gold special with road and Republic will increase from 8 to 12 arrows. It is especially important to have roads built on all trade specials before the Superhighways improvement to gain maximum benefit from this improvement.
The total trade output from terrain, road improvements, government, the Colossus (if present) and Superhighways make up a city's Base Trade. The Base Trade of a city is an important determinant of the two other aspects of trade: trade routes and caravan delivery bonuses.
I'll print it for use as a really practical reminder!
): roads (and rails) effect Base Trade (even without Superhighways) by adding 1 trade to the squares you mentionned above! 
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