How come railroad tracks that are welded do not buckle?

There is no "clickety-clack" on continuous welded rail (CWR). Continuous welded rail is layed in 1320' lengths (a quarter mile). Every quarter mile, one weld is required to bond these two ribbons together, on and on it goes. As far as buckling goes, you're on the right track (pun intended). Expansion, contraction and compression are the three major factors that keep me employed as a railroad worker. In high temperatures, we'll get "kinks" in the track that (I have seen) throw the tracks out of alignment up to 3 feet within a 50-foot distance. The rail looks like a piece of spaghetti. In the winter when the steel contracts, the rail will pop like a cannon and leave up to a 3" to 16" gap. Rail anchors help signifigantly reduce these effects. Anchors are the reason you don't see many "buckles" or rail breaks.

In an ideal world, the air temperature is measured over an extended period of time. Given the temperature range, the track will expand a predetermined amount and will retract a predetermined amount. The whole system of anchors, ballast(rocks) and ties prevents the track from buckling. The idea is that an anchor has a set amount that it will hold, the tie has a set amount it will hold, and the ballast surrounding the tie will hold a set amount. By adding up the retaining force of anchors, ties, and ballast, new rail is stretched or heated to the point where track buckles should not occur.

That being said, there there are many factors which can affect the holding force of the track. Including abnormally high or low temperatures, human error, train braking, age of anchors, condition of ties, etc., etc.

A link is provided to the Wikipedia article on continuous welded rail.