Between what speeds is dynamic braking on SD-40 locomotives most effective?

Dynamic braking on SD-40 locomotives is most effective in the range of 18-28 MPH because this speed range allows the dynamic brake system to operate optimally, providing a balance of braking force and control. At these speeds, the locomotive's traction motors, which are repurposed to act as generators during dynamic braking, can convert the kinetic energy of the moving train into electrical energy effectively. This process enhances braking performance by creating resistance, which slows the train without reliance on the traditional air brake system.

In contrast, at speeds below 18 MPH, the effectiveness of dynamic braking diminishes as the airflow across the locomotive and the resistance generated by the motors are not as significant. Similarly, at speeds above 28 MPH, dynamic braking may not sustain the necessary efficiency due to increased mechanical limitations and reduced effectiveness as the dynamics of the train changes with higher speeds. Thus, the optimal speed range for utilizing the dynamic brake system on the SD-40 is confirmed to be between 18 and 28 MPH.