It would be interesting to layer it with an elevation map.

We can almost make out the contours of the mountain ranges. For example if you look at France, there is like a big hole in the middle of the southern half of the country, that all train lines seem to avoid. That's the Massif Central mountain range, the largest of the country and a sparsely populated area. Same idea with the Apennines in Italy.

Compare to relatively flat countries like Germany and Poland where it is much more uniform.

You could use a population density map too, but if a low population area is flat, express trains tend to go straight through them instead of avoiding them, which give some spotty coverage rather than none at all as it is the case in mountainous areas.