Self-lapping

[...] Mr. Grierson remarks that on the L.P.T.B. tube stock in general the electro-pneumatic brake is in use, but that the new 1938 stock is fitted with the self-lapping electro-pneumatic brake, which is under the control of an automatic mercury-operated retardation controller when normal service stops are being made.

Thus, in the newer locomotives for the Great Northern line, the Woodward governors are adapted for air (not solenoid) control and continuous engine speed control through a self-lapping valve replaces the individual-speed steps, allowing operation in multiple with English Electric and Birmingham diesels.