Welcome to the forums!
Outerra uses a modern digital elevation map. The way this works is, a plane or a sattelite flies over an area and records the vertical elevation of the gound. (Don`t ask me how, I think they use lasers or something!)
These datasets are available for free or at a price, depenting on their detail.
The way Outerra works is, it takes a relatively low detail dataset and then "thinks" how the elevation would be if the dataset was more detailed. In other words, Outerra takes the main elevation and adds the details itself!
Now, because the Earth in Outerra is 1:1 scale, in other words, same size as the real planet, you can apply any other form of data produced by studying the Earth itself, on top of it.
So, for example, there is an open source map service called OpenStreetMaps. Its like Google Maps, but since its open source, you can download their dataset and use it on your own. For example, you choose to download the dataset containing all the roads in the world (well, most of them anyway, there are some missing, but its very rare. Usually small dirt roads and alleys) and apply it ontop of Outerra`s elevation dataset and Boom! You have the whole world with the roads on top of the terrain.
Then, lets say you go and download the dataset with all the railway lines in the world! Apply that on top of the two previous datasets and you have the terrain, the roads and the railways! Its layers upon layers of datasets.
That is of course when the Outerra SDK arives, or the developers choose to add it themselves.
I hope that helped a little and feel free to ask anything else you need!