Not so. There is currently about 260 000 km of railways in the US (of which about 150000 are 1st class), while there are 200 000 km in "Europe", which doesn't include places like the UK (over 15 000 km), so the two are very similar in terms of route length (adding in Canada and Mexico would make North America seem quite a bit bigger).

Edit: it occurs to me that the quoted figures are mostly route lengths, not track lengths. Since double tracking is much more prevalent in Europe than the US, I suspect the actual length of track is higher in Europe.