Looks like you are not getting an answer to this, the average situation is sort of right. The track has a design temperature at which the rails should be stress free, ie not in tension or compression. this will mean that the rails go into tension in the colder weather when the rail attempts to shrink and into compression in the summer when it tries to expand. This design temperature is usually above the mean temperature as the track structure can cope with tension more easily than compression.Tension is mostly contained by the tensile strength of the steel and the risk area is generally poor welds that may pull apart. Compression has to be constrained by the weight of the sleepers and the ballast shoulders preventing a buckle. This is the reason we sometimes get summer speed restrictions as any disturbance to the ballast from maintenance action or train dynamic forces makes a buckle, or 'sun kink' more likely. Note that new rail has to be welded in at the design temperature or lower as it can be hydraulically tensioned to the correct length but there is no way it can be shortened if too warm.
Keith Make friends in the hobby. Visit Garratt photos for the big steam lovers.