The Mystery of the Third Rail: Why Some Railroads Have Them

The third rail is a prominent feature in many subway systems and regional commuter trains. It is a conductor rail that supplies power to the electric motors of the trains, allowing them to run without overhead power lines. Today, we delve into the reasoning behind this interesting design choice and explore the various functions of the third rail.

What Exactly is a Third Rail?

A third rail is an electrical power supply line that runs parallel to the running rails. It is designed to provide direct current (DC) to electric trains, ensuring a smooth and efficient operation. Unlike overhead power lines used in some electric trains, the third rail is positioned on the side of the running rails, making it a more compact and space-efficient solution.

Motivation for Third Rail Usage

So, why do some railroads opt for a third rail instead of simply relying on running rails? The primary reason is the limited space inside the tunnel. Unlike overhead power lines, which require significant vertical clearance, third rails can fit within the confines of the tunnel, especially in urban areas where space is at a premium. This makes them a more practical choice for subways and elevated tracks.

Types of Third Rail Systems

There are a few different types of third rail systems used by different railroads:

Electric Power Supply: This is the most common type of third rail. It is simply used to supply electrical power to the trains. The trains are designed with a sliding contact shoe, which picks up the current from the live rail. Grip for Anti-Slip: Some third rails come with teeth or grip sections to help prevent trains from slipping back, particularly on steep inclines. These grip sections are typically located in the middle between two running rails. Meter and Broad Gauge Compatibility: Historically, in some regions, such as India, third rails were used to accommodate both meter gauge (1000 mm) and broad gauge (1350 mm) trains. The third rail would serve as a common power supply for both types of trains.

Operating Considerations

For operational safety, third rails are often equipped with shoe gear to allow the live rail to switch sides. This ensures that the power supply can avoid platforms or areas where it might be interrupted by junctions. In addition, third rails may have an earth return through the running rails or a fourth rail, with another shoe to complete the circuit.

Comparing Third Rails with Overhead Wires

While third rails are more common in subways and elevated tracks, overhead power lines are preferred for open tracks where there is a risk of people or animals crossing the tracks. Overhead wires are safer in such scenarios, as they are more visible and less likely to be touched accidentally.

Conclusion

From space-saving efficiency to safety and design flexibility, third rails serve a crucial role in the operation of many electric train systems. Whether it is for power supply, anti-slip assurance, or compatibility with different rail gauges, the third rail is a reliable and indispensable component in modern rail transportation.