A fluid transport tanker comprises a barrier member arranged for movement respectively between a first position and a second position so as to either prevent or allow connection of a delivery conduit to one or more outlet valves of the tanker, there being pneumatically operable locking means (1) for the barrier member comprising a locking element (2) to retain the member in its first position. The tanker further comprises pneumatic control means including first and second pneumatic switches (7,8) operable in response to movement of the locking element so as to enable the tanker to be filled and to prevent movement of the tanker, the first switch (7) being connected to a pneumatic pressure source via at least one manually operable master shut-down switch, and the second switch (8) being connected to the same or a different pneumatic source independently of the shut-down switch.