Any idea why Spain and Portugal have such small isometric zones? An obvious factor is the mountains in the North, but I'm surprised there isn't easier access between the Iberian peninsula and France

Yes, it is the mountains. Tunnels are very expensive. The French TGV lines only reach the Spanish border at Hendaye on the west coast and Perpignan on the east. The fast line to Barcelona is quite recent.

In northern Spain, there is a slow train line that connects Barcelona with Galicia called the "tren estrella," but it stops everywhere and uses old infrastructure, so it is slow. Traveling to Madrid is always fast with the newer AVE lines, and more are being built.

No idea about Portugal. I guess that it is the same situation, and those routes are covered by buses.

Well, Spain is fairly sparsely populated in the interior compared to France or Germany: https://maps-spain.com/maps-spain-geography/spain-population...

Then you have the Pyrenees between the Iberian peninsula and France. Aside from being mountainous, also fairly low population density, and rather poorer regions to the point that you'll find abandoned villages.

So, the only sensible connections are along the Mediterranean (Catalonia) or the Atlantic coast (Basque Country).

Another reason is that Spain has high-speed rails only since the nineties.

IRC, just in the recent years, there was a high-speed connection built to the Basque Country (from Madrid). From the Basque Country you can go to France with a light rail.

So, the only high-speed connection is along the Mediterranean.

In a way, it's a center/periphery problem.

France is prioritising connections from/to Paris, Spain from/to Madrid.

Unless either side improves the network at their side of the border, it also makes limited sense to improve yours.

The EU is funding that.

Severe lack of investment. While Spain was building one of the largest high-speed train networks in the world, we were closing down existing lines and building enormous highways.

The Iberian peninsula uses its own track gauge.