Which of the following is an example of a physical barrier?

A physical barrier is a tangible object that serves to prevent or restrict access, or to provide safety from hazards in a work environment, especially near train tracks. In this case, when a switch is blocked, it creates a physical obstruction that prevents any unintended movement of trains or equipment. This action is crucial in ensuring the safety of roadway workers by minimizing the risk of accidental train movements in their work zone.

The other options do not function as physical barriers. A train whistle is an auditory signal used to alert workers and others in the vicinity, while an emergency signal and a warning light are also forms of communication designed to convey information or warn of danger. However, none of these options physically obstruct anything or serve as a tangible means of preventing access, which is the defining characteristic of a physical barrier.