I’m not sure it’s legal to have standing passengers in high-speed trains. That’s why you have to buy your ticket before getting into the train, and showing the ticket to railway company personnel if requested. You have an assigned seat there.

In the short-distance trains you can get into the train only by passing a card through a rfid reader. Maybe you should compare this trains to UK ones.

Thus, Spain has less population than the UK, is less centralized than UK (where almost everything is in London) and Spain is much more mountainous. We can’t really compare these two countries.