The desirable length of an overtaking zone refers to the distance required for a train to safely overtake another train on a single track railway line. This is an important consideration in railway operations to ensure efficient and safe movement of trains.

According to the International Railway Congress (IRC) recommendation, the desirable length of an overtaking zone should be five times the overtaking sight distance. Let's understand why this recommendation is made:

1. Overtaking Sight Distance:
- Overtaking sight distance refers to the distance required for a following train to have a clear line of sight of the train being overtaken.
- It ensures that the following train has sufficient distance to safely overtake the slower train without any potential conflicts or risks.
- The overtaking sight distance depends on various factors such as train speeds, braking capabilities, and track conditions.

2. Importance of Length:
- The length of the overtaking zone plays a crucial role in determining the efficiency and safety of the overtaking maneuver.
- A longer overtaking zone allows the following train to complete the overtaking maneuver without any interruption or delay.
- It reduces the chances of conflicts with other trains and provides a comfortable margin for the overtaking train to safely pass the slower train.
- A longer overtaking zone also allows the overtaking train to maintain a safe distance from the slower train after completing the overtaking maneuver.

3. Five Times the Overtaking Sight Distance:
- The IRC recommendation of having the overtaking zone length equal to five times the overtaking sight distance aims to provide a significant margin of safety.
- By having a longer overtaking zone, the following train has ample space to complete the overtaking maneuver at a safe and comfortable distance.
- It minimizes the potential risks associated with overtaking, such as collision or derailment, by providing a larger buffer zone.
- The five times factor takes into account various operational and safety considerations, ensuring a sufficient length for overtaking on single track railway lines.

In conclusion, the IRC recommendation of having the desirable length of an overtaking zone equal to five times the overtaking sight distance ensures a safe and efficient overtaking maneuver on single track railway lines. This length provides a significant margin of safety and reduces the potential risks associated with overtaking.