The rail can only expand if it can move. If the rail is clamped down tightly to the sleepers, and the sleepers are tightly packed in the ballast, then it just doesn't expand. The only thing that happens is that stress increases / decreases as temperature changes.
The pre-tensioning is only to ensure that the average tension in rail in the course of an average year is in a certain range.
The rail can only expand if it can move. If the rail is clamped down tightly to the sleepers, and the sleepers are tightly packed in the ballast, then it just doesn't expand. The only thing that happens is that stress increases / decreases as temperature changes.
The pre-tensioning is only to ensure that the average tension in rail in the course of an average year is in a certain range.