Great question Hutch.

I'm not an authority on this (or most other) subject, so I search for answers online, in this case for [Rail Corrugation].

For an overview, I'll typically head for the Wikipedia item. In this case, the Wikipedia item is quite brief and is titled "Roaring Rails". Here's the relevant text, which is almost all of the item: "Roaring rails are very short wave corrugations that range from 25 to 75 millimeters in length. It is commonly associated with high speed passenger, transit, and light axle load railroad operations.
Causes[edit]

It is generally accepted that a few distinct causes lie behind different wavelengths of railroad corrugation. One study[citation needed] indicates that the specific short-wave railroad deformity is mainly caused by pinned-pinned resonance, in which the rail vibrates as a fixed beam, as if pinned between periodically placed sleepers. The dynamic train-track interaction that causes fixed frequency vibrations at high speeds, commonly observed in light load metro operations, and the anti-resonance caused by the pinning of the rails on sleepers, causes deformation and the "roaring" corrugation of the rails."

The 2002-2006 EU report provides a description of the problem causes and solutions.

I hope this thread will grow with good comments.