The gaps do give some amount of play. The trouble is that once the temperature exceeds the design maximum the gaps close up completely and all that expansion fo

The gaps do give some amount of play. The trouble is that once the temperature exceeds the design maximum the gaps close up completely and all that expansion force is directed into hard metal-on-metal contact, at which point something has to give. Jointed rail tracks are not designed to handle those forces without buckling because the whole point of having the joints is so that they don't have to.