Word of advice: please note that this might not be a safe way of doing things, but I believe this is the way you can change your tracks in the editor, after having laid some tracks already. In this example I have used the KWG high speed track. I have laid those tracks first: Then I select the Ribbon Loft Descriptor to the new track lofts I want to replace. In this case I'll replace them to our custom tracks. I think for the sake of it, it should be also important to choose the right track rule for the new tracks. I have laid the new track next to the one I want to replace just to show you that I'm already using a different track on the track properties. Next, I go into select mode, and I need to have access to the track proxys. Then I right click on that proxy and choose the option to "Replace Loft Descriptor with" And the track gets replaced. Another thing worth of mentioning is that this does NOT replace the track rule that was previously used. This must be changed manually. You can do so by toggling the ribbons and then change the track rule on the right side options.
An example of what you can do with this is to change the type of tracks. What I did here was to cut the already laid track where I needed to change the type of sleepers I wanted, and then just select the track with the new sleepers on the Track Properties, used the Replace Loft Descriptor" mode as described above and exchanged the track as I needed to.
But is there a way to change the track rule? I know selecting the proxy ribbon is suppose to change it, but it does not.