It's silly how moving an army or a settler across the map takes ages literally. There should be a fast travel mechanic.
The first possibility is the rebase mission, like for planes. Units could be rebased to remote cities and forts connected by roads. The distance could be limited by technology. Railroads should have unlimited range. It won't be game breaking as it will only be possible to rebase 1 unit per [destination] city/fort per turn. And maybe it should cost some gold.
The second is sea transportation. Similar to the Great Admiral's rebase, but not only to another port but any shore location in a certain travel radius. Enemy ships may block passage in quite an area (say, in the radius of their moves), or sink any transports in their ZOC. The transport capacity is limited by the number of harbors, technology and geography (how many ports can reach a particular destination).