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 GeoJSON?

The format web maps speak - points, lines and polygons as plain JSON, with arbitrary properties hung off each feature. Leaflet, Mapbox and every mapping library read it natively, which makes it the usual destination for map data that came from somewhere else.