Hey there,
Glad you found this useful :). Yes, you can add your own custom meshes for the intersections and road. In the case of the road, simply change the ‘pavement’ mesh with your rails mesh under the 1 Lane Defaults and/or 2 Lane Defaults. You can also change the pavement mesh on a segment basis - open the segment and change the mesh with your rail road mesh.
As far as performance - make sure you make the edits with Edit Mode turned on! If you are still having some issues, you can use Spline-Only mode, or simply create multiple Roads and connect them (instead of one very long road). The second option is probably better. Create your first road and then press ‘Spawn Actor’ to spawn another road at the end. This should help with performance.
Hope this helps!