Sentence:  Bogie <WSD>hunting</WSD> is not caused by some sort of periodic disturbance but by dynamic instability; the oscillatory system is not the bogies alone but the complete assembly of bogie-plus-body; and variations in track rigidity do not affect the nature of the motion, only its intensity. 
Meaning of the word marked with <WSD> tags:  Fluctuating around a central value without stabilizing.