earthsong9405:

Midna was able to pass into the Light Realm without the Spirit Train and tracks before it was even built. The Mirror was the easiest way to travel between the two realms but it wasn’t the only way by any means. I’d imagine she simply found another portal (or hell, made another one) to use as her means of coming and going to the Light Realm, and that’s how she came into contact with the Light Realm once more. I think she’d have to actively teleport herself to the realms through the portals though so it’d be physically taxing on her, especially if she tried to bring more than a few people with her (which I’m sure she did in order to better discuss transportation and bridging the gap once again between the two realms). 

Those long talks of alliance and officially giving the two realms a “main entrance” to each other, as it were, finally lead them to the construction and invention of the Spirit Train and the eventual creation of the AI creatures Locomo, of which pilot and maintain the train. o3o