Route km or track km? Yorkie might claim that the Swiss railways are more intensively used than ours but I've a feeling that if you looked at the occupancy rate of a given track circuit in the throat of a major station, the British one is occupied more, simply because of the lack of grade separation and the use of ladders or even sections of single track to save money when BR "rationalised" in the 1980s.
From the report:
Network usage intensity is an indicator of the overall occupancy of the railway network, as it measures the number
of train-km per route-km per day. Since the measure is done for the whole country, it does not account for how
usage can vary between different regions within a country
Which confirms it's route km, which means that if there's lots of quad tracks in a country or would show up as more intensively used than if they were mostly twin tracks (or even hard large amounts of single tracks).