Question Author
One result of the franchise bidding process is that moving entire fleets of trains between lines is most easily done when a franchise expires. Often the introduction of new trains for one particular line can lead to a "cascade", as the older fleet moves to another line to replace an even older fleet, and so on. In the days of British Rail, railway managers were free to move rolling stock around to meet demand.
I reckon that the answer is CASCADES.
Thank you to everyone for your time. You learn something every day!