That is actually not true, train prices (in Germany) are fluctuating based on demand as well. But you are not tracked individually and generally the prices are simply rising the closer the travel date gets. So the situation is better than with air travel.

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 is know in advance, and the maximum price is always fixed. So quite different.

> 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.

But the parent comment explicitly mentioned that case, it's not fluctuation as it is implied to be quite predictable.

> there is just a certain amount of lower fares (going up in stages, the closer you get to the travel date...

Well I take almost every week the ICE from Frankfurt to Paris and it always costs the same: 125,40€. No matter how in advance, no matter the day I travel. Try that with a flight... Even if this is not true for all trips, for those that it is, it is another advantage.

I just searched for a random Thursday in August and found a ticket for 54,90 EUR.

But the real question is: does a booking in that timeslot cost the same every Thursday, with a ficed price? Or does it fluctuate?

I don't understand how that's "the real question". It's a Sparpreis. They fluctuate. You can buy a full price ticket that doesn't fluctuate if you want to.

>train prices (in Germany) are fluctuating based on demand as well

That's true for saving fares (Sparangebote). The train price for Flexpreis (an offer without a binding to a certain train) does not change that often.