Why do some railway rails have small gaps between their ends?

A steel rail becomes slightly longer when its temperature rises. In track made from shorter connected rails, small gaps can leave room for that expansion. The gap changes as the rail changes length.

If a warming rail is prevented from becoming longer, compressive force builds within it. That is different from the support that holds the track against sideways movement. If the compression becomes too large for that sideways restraint, the track can buckle. A small thermal change along a long rail can therefore create a significant structural problem.

Most modern main-line track uses long continuously welded rails instead of a gap at every short section. Engineers manage the resulting thermal forces through controlled installation conditions, fastenings, sleepers and the surrounding track structure.

Neither arrangement makes temperature irrelevant. The important question is whether expansion appears mainly as movement or as force within a restrained material. Rail design handles both while keeping wheels supported along a predictable route.