A rail fare that was one price yesterday and noticeably higher today has not drifted. It has moved from one inventory bucket to the next, which is a step and not a slope.
Inventory is divided into priced buckets
A train's seats are split into groups, each with its own price. The cheapest group is offered first, and when it sells out the next price becomes the lowest available.
The buckets are set when the train is opened for sale, based on the expected demand curve for that date and route.
Nothing about the seat differs between buckets. The fare difference reflects when it was bought relative to the fill of the train.
Release happens in a schedule
Seats do not all go on sale at once. Operators release inventory in stages, holding some back for later demand and for connecting itineraries.
This is why a sold-out train sometimes shows availability weeks later, and why watching a single fare over time gives a misleading picture.
It also means the first day of sale is usually the cheapest, since the lowest bucket is fully available and untouched.
Sleeping accommodations price differently
A room charge on a long distance train is usually a separate amount added to a rail fare, and it moves on its own bucket structure.
Because the number of rooms per train is small and fixed, that inventory exhausts faster and the steps between buckets are larger.
The rail fare component for a room passenger typically prices at a higher level as well, so both parts of the total move together as the train fills.
Demand is estimated, not observed
Bucket sizes are set from historical patterns for that day of week and season, so an unexpected event on the route produces prices that lag actual demand.
Operators can adjust, but adjustment is manual and coarse. This is why fares occasionally look wrong in both directions.
It also means a quiet date genuinely holds its low bucket, rather than the price rising simply because departure is approaching.
What this implies for buying
Booking early works because it is the mechanism, not a marketing claim. The lowest bucket exists and is finite.
Flexible dates help more on rail than the schedule suggests, since adjacent days often sit in entirely different buckets on the same route.
Refund and change rules usually tighten as the fare falls, so the cheapest bucket is also the least flexible, which is the real trade being made.