A "Call-On" should NOT be given for?

A "Call-On" is a permission given to a train to proceed under certain conditions, usually when it is necessary to clear an obstruction or operate within limited visibility. However, issuing a "Call-On" for an obstruction on the road bed would pose significant safety risks.

When there is an obstruction, it can interfere with the safe passage of trains. Allowing a train to proceed in such a scenario without adequate visibility or clearance could result in accidents or derailments. Therefore, it is crucial to ensure that the path is completely clear and safe for movement before granting such permissions.

The other options present scenarios where a "Call-On" could be appropriate under the right conditions. For instance, a train that is moving behind schedule may need to be maneuvered efficiently to minimize delays, switch position changes typically do not create immediate safety hazards, and a clear track signal indicates that it is safe for a train to follow.

In summary, a "Call-On" should not be given when there is an obstruction on the road bed, as this compromises the safety of train operations, making it the appropriate choice in this context.