Step 1: Define rail network density. This refers to the length of railway lines per unit of land area (e.g., km of rail per 100 sq km).
Step 2: Compare the continents. Europe, with its relatively small land area, high population density, and long history of industrialization, has developed an exceptionally dense and integrated rail network connecting its numerous cities and towns. While North America and Asia have longer total rail lengths, their vast land areas mean their density is lower than Europe's.