Approach locking of signals was and is still not a universal feature of mechanical locking in Absolute Block areas, especially at smaller boxes that have been less heavily modernised over the years. Typically it requires a B position electric lock on the signal lever concerned, or "back-lock" as it's sometimes known, that prevents the lever from going fully back to the normal position for a time, so preventing the release of any mechanical interlocking applied to other levers in the frame such as level crossing locks, points and FPLs, and other directly opposing signals. Alternatively a point or crossing lock lever might have a timer applied to it's full N or R position electrical lock (according to which lever position locks the gates closed in the case of a level crossing). Some larger boxes still extant, or surviving until very recently, along the WCML were heavily modernised in the 1960s with new colour light TCB layouts having full electrical controls applied to their mechanical frames equivalent to modern relay interlockings, including sectional route locking and release, approach locking etc.