Final answer:
The voltage carried by DC conductor rails varies, with common values being 500, 750, 1,000, or 1,500 volts, depending on the design and requirements of the light-rail system in place.
Step-by-step explanation:
The voltage carried by DC conductor rails for light-rail systems such as subways and trams can vary depending on the system's design and requirements. Based on the options provided, which include 500 volts, 750 volts, 1,000 volts, and 1,500 volts, light-rail systems typically operate within this range. However, without specific context it is challenging to provide a definitive answer as different transit systems can utilize different standards. For instance, some subway systems may operate on 750 volts DC while others might use 1,500 volts DC. In the instances provided, measures of electric fields and forces on wires carrying current are examples of the application of physics in understanding electrical systems and do not directly inform the typical voltages for DC conductor rails.