If a train operator detects a strong acrid smell and arcing, what is the most likely cause?

The presence of a strong acrid smell alongside the observation of arcing strongly indicates that there is likely an issue with the electrical components involved in the operation of the train. A grounded shoe beam is a component connecting the train's electrical system to the rail, and if it is compromised or improperly fixed, it can create a situation where there is an electrical fault. The grounding can lead to arcing as the electrical current jumps due to improper connections or wear, generating both heat and an unpleasant smell.

This scenario directly correlates with the signs given—in particular, the acrid smell often results from electrical arcs as they burn material, and the visual of arcing indicates malfunctioning electrical flow, frequently linked to grounded components. The other options may relate to various electrical concerns but do not specifically connect both the odor and arcing symptom as clearly as a grounded shoe beam does.