This pattern is true and passenger kilometers represent it just fine. There is no need to use the how often you use the train metric. Note that my two examples were there to explain the metric, not actual factual examples.

As an actual example: I take my bike to work and dont own a car, so my modal split is mostly trains because of longer distance trips, but I use the bike far more often. Frequencies only make sense if each occurrence is very similar (in quantity). For example: How often does one eat meat? Each meal roughly contains the same amount of meat (may be factor two or three difference). Here frequencies make more sense as more detailed statistics dont actually give more insights.