Rails are made of metal, and it expands and contracts as the temperature outside changes. If rails were to exactly butt up against each other, their expansion could cause their shape to change or cause the rail to buckle. Having some play in the joint and bolt-hole allows for some �give� in this expansion/contraction. You see the same effect in sidewalks where the concrete is formed in sections with gaps in between�normal expansion of concrete could cause it to crack if there is no �give� between the pieces.
