Which of the following is true about the running rails' voltage?

The statement that the voltage of the running rails can vary from 0 to 12,000V accurately reflects the operational conditions found in rail systems. In electrified rail networks, especially those involving trains that utilize various overhead power systems or traction power substations, the voltage can fluctuate during operation.

When trains are not in motion or when systems are inactive, the voltage can drop to 0V. Additionally, during operation, especially in high-speed or heavy-haul lines, the voltage can increase significantly, reaching high levels such as 12,000V in certain areas designed for specific electrified train operations. This variability is crucial for understanding how rail systems operate under different conditions and loads.

By understanding the potential voltage extremes, linemen can take appropriate safety precautions and make informed decisions when working in proximity to rail systems. The context of how this voltage is applied and managed in a rail environment highlights the importance of flexibility in voltage management, which is consistent with the correct answer.