Muni streetcar lines exhibit some corrugation in certain spots. You won't generally see it at stops as they wear down faster.

Cable car lines also show corrugation, which would seem to affirm the lateral slip theory as cable-hauled should not have any "longitudinal" slip. I theorize, with zero proof, that rigid as opposed to resilient encourages vibration and thus worsens corrugation.

I believe that along with mechanical pounding and hammering steel molecules are traveling ever so slightly along the rail and depositing in a wave form. Pure speculation on my part.