Sentence:  Provision was made for this <WSD>cascade</WSD> of units when TfL <small>[Transport for London]</small> exercised an option in its order for Class 710s from Bombardier for an extra 6x5-car units and 3x4-car units: these would be used on the North London line and release '378s' for the East London line. 
Meaning of the word marked with <WSD> tags:  A stream or sequence of a thing or things occurring as if falling like a cascade.