What is TCX?
TCX is Garmin's training format: a GPS track with heart rate, cadence and lap structure wrapped around it. FileSorcery reads it and converts out. Writing one is deliberately not offered, because a schema-valid TCX needs activity timestamps that a converted file cannot honestly invent.
What is GeoJSON?
GeoJSON is what geographic data looks like when it stops pretending not to be JSON. It is the lingua franca of web mapping - Leaflet, Mapbox, D3 and most spatial databases read it directly - and its one famous trap is coordinate order: longitude first, then latitude.