Have you ever wanted to react to events taking place on a web page, but they aren't capital-e Events that you can catch and handle? I was in this situation recently, where I wanted to grab the current video and chapter titles to display on my Twitch stream. YouTube isn't going to provide you with any convenient events to handle, but you can use the magical MutationObserver in order to listen for changes on particular elements! As it turns out, this was just what I needed.
I used the DOM inspector in Chrome's developer tools in order to drill through the document and find the elements I needed to focus on. (In a page like YouTube, this can be a bit difficult to manage, but have patience.) Setting up two MutationObservers--one for the video title and one for the chapter title--I was able to effectively generate my own events. This allowed me to catch the track change as it happens without using any yucky polling mechanisms or other less-than-elegant solutions.
The project I'm using this in is yt-siren; the companion UserScript has the logic mentioned above, and it's been working wonderfully thus far. Maybe you will be able to build something neat using the same pattern. Good luck!