When in route editor of Rail Simulator you can navigate in 3D space using the arrow keys or the 2D map. Click the map icon in the compass window to toggle between 2D and 3D views.
Move about the 2D map by click and dragging. To center the map and enter the current coordinates press Ctrl and left click. When you switch back to 3D your view will be those in the compass coordinates.
To adjust camera to ground level press Ctrl+Shift+Home.