On a side note: yes, some of the regulations on the de-facto monopoly railway company in Spain, RENFE, are simply ridiculous.
These days, for travelling medium to long distances the only option tends to be high-speed trains (in one of its 'flavours', whether AVE, AVLO, ALVIA, etc) Regulations on high-speed trains are absurdly strict, even draconian, and simple things like carrying a bike or travelling with a dog or with two or more large suitcases can become cumbersome and even impossible.
Example: if you intend to travel with your dog and he is over 10kg you CAN'T travel on a high-speed train. Period. Not with a special transport cage, not paying an extra ticket no nothing. if you can't reach your destination by MD (medium distance) train, you better change your plan because there's no alternative to travel with medium/large dogs, not on a bus not on a rental car (most companies specifically indicate "no dogs" on their rental contracts).