> Except they don't fluctuate, there is just a certain amount of lower fares (going up in stages, the closer you get to the travel date and depending if all tickets of that category were booked already).

This isn't true, or at least that's not true in a general sense. IIRC ticket prices of all high-speed services offered by Spain's high-speed train operator, Renfe, increase as we get close to the travel day. In some cases, I assume due to low demand, sometimes prices drop a couple of weeks to the travel day. I assume all prices follow a predetermined progression accompanied by ad-hoc adjustments depending on demand levels.

Source: I've scraped price data out of Renfe's site for fun.