In subway systems, what is the primary purpose of connecting heavy copper bonds across the joints of track rails?

The primary purpose of connecting heavy copper bonds across the joints of track rails is to determine and maintain electrical resistance. These bonds play a crucial role in ensuring the electrical continuity of the track system. By connecting the rails with copper bonds, the electrical resistance across joints is minimized, allowing for efficient electrical signaling and train operations. This is particularly important in systems where the rails are used not only for structural support but also for the conduction of electrical currents, such as in electrified transit systems.

The other options do not align with the main function of copper bonds. Aesthetics are not a consideration for this technical application, and reducing noise, while beneficial in some contexts, is not the primary function of these bonds. Additionally, while rail strength is important, the role of copper bonds is specifically related to electrical conductivity rather than structural reinforcement of the rail itself. Therefore, the focus on minimizing electrical resistance through the use of these heavy copper bonds is what makes this answer correct.