I would make a new layer in your landscape material using your ballast texture, and paint the ballast texture by hand. Other than making a new static mesh or loft to use as the track bed, it's probably the easiest way. The landscape material tutorial at the end of DonnyDave's guide (https://forums.dovetailgames.com/threads/unofficial-terrain-creation-guide-for-beginners.75275/) is a great resource for making a layered landscape texture.
well, I'm note sure why to be honest, but it looks like you've created multiple landscapes. You can probably just delete the extras, so long as you're careful not to delete the one that has your terrain! It's kind of a different thing than I was suggesting, though. I don't mean to be rude, but may I ask if this is your first time using an UnrealEditor-based editor by any chance? I've found that Youtube has tutorials on just about every aspect of using UE4. The part at the very end of DonnyDave's document (the part about painting materials) is what I was talking about earlier, but the whole document is really worth following through. In any case, save often, make backups , and best of luck!
