it's high density urban centers with empty countryside in between. the problem with low density is that many areas don't have enough people close enough to any station to be able to get there without a car

In Spain, there's usually no "density gradient" between the core of a city and its suburbs.

The core of the city is dense, the suburbs are equally dense, and the conurbation's edge is a street which has a five-story building on one side and farmland on the other. Outside the conurbation, it's primarily farmlands and forests, with even small towns following a similar pattern.

That is, the density of a given area is either somewhere in the thousands of people per square kilometer, or in single digits.

(Some cities do break this pattern to some extent, but it's mostly isolated phenomena, such as the suburbs of Madrid around the A-6, or the continuum of housing along the Costa del Sol)