Define AC traction and provide an example.

• A. Traction powered by direct current, typically 750V DC.
• B. Traction powered by alternating current, typically using a feeder from a substation and an inverter for traction motors; example 25 kV AC.
• C. A traction system that uses both DC and AC for propulsion.
• D. A signal-based traction system.

Answer: (B)

AC traction means the train is powered by alternating current, delivered through the railway’s overhead lines or a third rail and controlled with onboard power electronics. In this setup, power is brought from substations via feeders, and the traction motors are driven by an inverter that converts the incoming AC into the right form (voltage and frequency) for propulsion, enabling smooth speed control. A common real-world example is 25 kV AC, where the overhead line carries high-voltage AC and the system uses substation feeders and inverters to drive AC traction motors. This combination—AC supply, feeder-substation distribution, and inverter-controlled motors—is what defines AC traction. The other descriptions don’t fit because they describe DC traction, a mixed DC/AC approach, or something unrelated to propulsion systems.