What is TCX?

Garmin's training format, and far more than a route: heart rate, cadence, laps, calories, all sampled through the workout. Converting it to anything else means deciding which of that to lose, because GPX and GeoJSON have nowhere to put most of it.

What is KML?

Google Earth's format, and the reason it looks good - unlike GPX or GeoJSON it carries styling, so lines have colours and placemarks have icons. Still XML underneath, and still the thing to convert to when someone wants to see a route draped over terrain.