It is the basic pricing economics of supply and demand. If you don't like the price travel on the 3rd or 4th of January. Or better still you should have got your act together and booked earlier, if you were unable to confirm your plans earlier then you can hardly blame the train company for selling the seat earlier to another passenger at a lower price.
I could go onto explain that train, airline, hotel etc pricing and overbooking is almost as Marxist in it principles as it is capitalist. It is in neither in the consumers' or company's interests to have static non dynamic pricing, and not to overbook. This is one example where the fat cats really do support the poor.
Oh and that train on the 2nd of Jan may depart with empty seats, and most probably will! Companies with highly perishable* products do not maximise their profits by selling every product it has - the Marxism bit falls over here a little.
I worked in airline revenue management for many years
* pretty much every consumer product is perishable