A Direction of Flow arrow, sometimes abbreviated to DOF, is an indication to the signaller that a section of line has been locked in a particular direction. This means that signals can only be cleared in the direction that the DOF arrow indicates.
They are usually provided for longer stretches of line signalled for bidirectional running, but can sometimes also be found on shorter stretches of bi-directionally signalled lines around junction or station areas or on simple single line sections.
In this example from Brighton sim, the Direction of Flow arrows are illuminated for the normal direction of travel on both lines:
The exact method of operation can vary from signal box to signal box. In some cases (e.g. on LTS ), the arrows for both directions remain unlit until a route has been set into the bidirectional section, in other cases (e.g. Brighton ), they are always lit for the normal direction of travel unless a route in the opposite direction been set. There is also an alternative method of operation (e.g. on Rugby North ) where the signaller has to manually reverse the DOF arrows before being able to set a route in the other direction.
Last edited by GeoffM on 15/09/2016 at 03:00