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You are correct that steel will expand in every direction as it warms, however, due to the shape of rail, it's largest natural expansion/contraction would be lengthwise. Thus they are forcing it to behave differently than the pyhsics of it's construction predispose it to. As to the gauge, there is enough "slop" in the gauge to account for the expansion/contraction of the rail. I don't remember the exact specs, but there is ample room to acount for fractions of an inch in expansion. I have never seen it lift the track structure although I'm sure it would be possible, what usually happens is a "sun kink" or thermal misalignment as the railroad likes to call it. The track bows out to one side or the other. Sometimes they are a series of small "waves" (look at a stretch of straight track on a real hot day and you can usually see this) or sometimes it's a big blow out to one side or the other (usually causes a derailment if the train hits it fast enough). I saw one on a stretch of double track that had bowed all the way over to and sort of on top of the track next to it! (got lucky we didn't have a collision there!) Also, all these same forces have the opposite affect in the cold, contracting the rail and putting it into extreme tension, that's why you have so many broken rails in the winter, it just pulls the steel apart or if you happen to have some joints it breaks the bolts (sometimes breaks the joint bars themselves too).
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