First of all, in the images shown in that article the rail clips are rigidly set in concrete, and the rails are clipped in place. With enough force they can slide longitudinally in the clips.

Second, in most modern rail installations the track is pre-tensioned to deal with thermal expansion. The track is stretched with a hydraulic tool and welded while under tension, so thermal expansion just means reduced tension. This is why trains on modern rail lines don't make the familiar click-clack sound.

(How much tension is applied depends on the country and weather - you can choose between tolerating high temperatures and tolerating low temperatures. This is why in the UK, there are sometimes train stoppages in very hot weather)