Running TSS (rTSS) is calculated using Normalized Graded Pace (NGP), which is intended to account for changes in pace when going up or down hill. Under most conditions and terrain, this is enough to provide accurate numbers while trail running. In some situations, the NGP algorithm can fully account for the other factors that can affect pace like the surface, wind, short steep pitches up and down that GPS can't detect well, and the rockiness/rootiness/general technical difficulty of trails. In this case, we recommend changing the TSS calculation for trail runs to heart rate based TSS (hrTSS) instead of pace based TSS to see if that more accurately reflects the overall effort and training load of that workout. For this to work correctly you will need to make sure that you have heart rate data in that workout and you have the correct threshold heart rate for running set in your account settings.
If you don't have heart rate data for a given trail running workout or race you can also estimate and manually correct the TSS value for that activity using the following as a guide.
Easy = 30 TSS/hour
Moderate = 60 TSS/hour
Hard = 90 TSS/hour