Definition of "cut-off"

[sense 4 at cutoff] I should doubt if the consumption exceeded 30lb per mile, although it must be remembered that the regulator and cut-off were virtually unchanged from Etaples to Amiens, while the speed remained constant at 100 k.p.h. (62.5 m.p.h.) for mile after mile.

1961 March, ""Balmore"", “Driving and firing modern French steam locomotives”, in Trains Illustrated, page 150