Advance rail fares are frequently gone weeks before departure on some trains and still available days before on others. The pattern follows from quotas attached to individual services rather than to the calendar.
Fares are released in buckets, not tiers
An operator assigns a limited number of seats on each train to the lowest fare, a further tranche to the next level, and so on up to the flexible price.
When the cheapest allocation sells out, the system offers the next one. The price rises because a quota has been exhausted, not because a countdown reached a threshold.
This is why the same journey can cost very different amounts on two departures an hour apart. Their quotas are independent and sell at different rates.
The purpose is to fill trains that would run anyway
A scheduled train's cost is largely fixed. Crew, energy, path charges and rolling stock are committed whether the seats are occupied or empty.
Selling a marginal seat cheaply therefore adds revenue at very little extra cost, provided that seat would otherwise have travelled empty.
The discipline is to avoid selling cheaply to someone who would have paid the full fare, which is what the quota system is designed to prevent.
Conditions do the sorting
Cheap fares are tied to a specific train and are non-refundable or heavily restricted, which makes them unattractive to travellers whose plans might change.
Business travellers value flexibility highly and therefore self-select into the expensive, changeable fare rather than accepting a binding commitment months ahead.
The restriction is the mechanism. Without it the operator could not offer a low price to one passenger while holding a high one for another on the same train.
Booking horizons vary by operator
Tickets generally go on sale once a timetable is confirmed, and that horizon differs between countries and between domestic and cross-border services.
Cross-border journeys often open later because two or more operators must agree the through fare, which is why an international leg can lag the domestic one.
Splitting a journey into separate tickets sometimes reveals cheaper quotas, though it removes the protection a single through ticket provides if a connection is missed.
What actually improves the odds
Flexibility about departure time helps more than booking earlier, because quotas are allocated per train and the unpopular departures retain cheap seats longest.
Travelling outside commuter peaks and away from Friday and Sunday evenings addresses the same effect from the demand side.
Watching a specific train rather than a route also helps, since availability moves independently and a released seat can reappear at a lower fare after a cancellation.