Which interlockings cannot be removed from service?

Interlockings are critical components of railway signaling systems, designed to prevent conflicting movements through junctions or crossings. The presence of certain features, such as movable bridges or railroad crossings at grade, creates unique safety concerns and operational requirements.

When interlockings include a movable bridge, their removal from service would compromise the ability to control train movements safely when trains are traveling over or approaching the bridge. These interlockings ensure that a train does not enter an area where it could potentially collide with another train or be misdirected onto an unsafe track. Similarly, railroad crossings at grade involve intersection points where vehicles and trains share the same level, necessitating careful control to avoid accidents.

Thus, interlockings that incorporate these features are essential for maintaining safety protocols and operational effectiveness in rail traffic. Removing them would undermine these critical safeguards, highlighting why this choice is the correct answer.