I don't know why mountains are impassable in Civ4. This is not entirely accurate, as many mountain ranges throughout the world have been used as passages for centuries. I would understand making them passable only by certain units; for instance scouts and explorers could use them to increase their viewing distances, with a movement penalty of course. Workers should be able to pass through mountains to build roads, rail, and mines, etc.
It could also be possible to make the mountains impassable only until some kind of tech is discovered, such as map making or engineering. Then the workers could make roads or rail and allow units to pass only by that route.
Another suggestion that I read in a similar thread was to create two types of mountains. Passable and Impassable. This would add to the strategy of the game in that there would be mountain pass areas that could be used as ambush points, etc.
It could also be possible to make the mountains impassable only until some kind of tech is discovered, such as map making or engineering. Then the workers could make roads or rail and allow units to pass only by that route.
Another suggestion that I read in a similar thread was to create two types of mountains. Passable and Impassable. This would add to the strategy of the game in that there would be mountain pass areas that could be used as ambush points, etc.