What is a third rail and where is it used?

• A. A low or running rail that provides electric power to trains via contact with collectors, used in some urban rail systems.
• B. An overhead wire.
• C. A track switch mechanism.
• D. A signal indicator.

Answer: (B)

The concept being tested is how electric trains receive their power. A third rail is a low, running conductor placed alongside or between the train tracks that provides electric power to trains through contact with collectors or shoes on the train. This setup is common in many urban rail systems and subways because it keeps the power equipment at or near track level, avoiding the need for tall overhead structures. The third rail typically carries DC power (often in the 600–750 V range) and must be insulated and protected for safety.

The other descriptions describe different features: an overhead wire is part of a different power collection system (catenary) used by many intercity and some regional lines and requires a pantograph, while a track switch and a signal indicator serve routing and signaling functions, not power delivery. So the best description of a third rail is the low conductor alongside or between the rails that trains contact for power.