Yes, we can get railways data from OSM. It's easy.
But the need some asset, which can work with dynamic railways.
For example, for highways is: RoadArchitect, EasyRoads3D, SplineBend.

For example, is it possible to have an OnlineMap running and have RealWorldTerrain overlaying trees, grass, buildings, rivers, roads?

I think thats the main thing really. If Online Maps uses regular Unity Terrains, then I think it should be possible, otherwise maybe its not...

Many thanks!

You can show on the map some object which is in the scene, which contains terrains created in Real World Terrain.
About integration indicated in description of Online Map, because it asset contains integration script.

Unfortunately, the current version Real World Terrain does not support the railways and ferries.
The problem is that we do not know a good asset for it.
If you know a good asset for the railways or ferries, please tell us.
Theoretically, we can add the generation of railways using SplineBend.

Yes, you can do it.
You need to have both assets.
You create a scene with the map and the other scenes with terrains.
You load a scene with terrain, when necessary, using Application.LoadLevelAsync.

I need an asset for a real world simulation that Im going to be working on, and I cant decide which of your two assets I need. Both look great, and Im wondering how / if they work well together? On this page http://infinity-code.com/en/products/online-maps it mentions 'Integration with Real World Terrain', but on the Real World Terrain page it doesnt mention Online Maps at all...

So Im wondering, how well do they play together?

RealWorldTerrain - I like the ability to add OpenStreetMap data, roads, and rivers. (is there railways and ferries too?)

Online Maps - I like the navigational elements, specifically the camera zoom to 3D markers, loading/unloading terrains on the fly, and hiding / showing labels, etc.

My question is, can I use both assets together? I want to be able to use Online Maps for allowing my users to fly over the map, loading terrains as they are needed, showing/hiding labels, clicking on markers to navigate between areas of the map. Plus, I want the map to be a real living world - so when they zoom down in closer it shows the detailed terrain assets - trees, rivers, buildings, grass and other objects.

Is this possible with Online Maps and Real World Terrain? Do I even need both assets for this?

Many thanks!