Harmonic Rock is a condition that can occure in certain cars depending on weight(particularly large covered hoppers loaded with grain). It's not the condition of the rail, it will accure on CWR(although a rough rail section doesn't help.) Accures between 14 mph and 21 mph. Fed. testing in Colorado discovered the condition and found that the weight of these cars, the speed between 14 and 21, combined with the earths rotation(NO BULL) caused "Harmonic" rocking action. It's simular to the natural actions of whirl pools and cyclones which always have a counterclockwise rotation. As engineers, we were trained to avoid operating between these speeds whenever possible. Either get it above 21mph, as soon as safely possible or stay below 14mph.
Harleys and EMDs, some of my favorite rides