What voltage levels are commonly used for DC traction systems?

DC traction systems typically utilize voltage levels that are most effective for the specific needs of railways and mass transit systems. The correct option mentions 750V, 1500V, or 3000V, which are standard voltage levels commonly employed in various electrical railways worldwide.

The 750V level is frequently used in urban and suburban transport systems, such as trams and light rail, due to its balance between efficiency and safety. The 1500V level is common in certain regional rail systems, while 3000V is often found in high-speed rail applications, allowing for reduced current and increased distance between substations.

This wide range provides flexibility and ensures that systems are optimized for different operational requirements. In contrast, other choices include voltage levels that either do not commonly exist in traction systems or are less practical for the efficient operation of such networks. For instance, while the 1000V mentioned in one option does exist in some applications, it is not as prevalent or standard as those in the correct selection.