Minimum Composite Sleeper Index prescribed on Indian Railways for a track sleeper is 783.

Explanation:
The Composite Sleeper Index (CSI) is a measure of the overall performance and durability of a railway sleeper. It takes into account various factors such as load-carrying capacity, resistance to wear and tear, and life expectancy of the sleeper.

The minimum Composite Sleeper Index prescribed on Indian Railways for a track sleeper is 783. This means that any sleeper used on Indian Railways must have a Composite Sleeper Index of at least 783 to ensure adequate performance and durability.

The Composite Sleeper Index is determined through various tests and evaluations conducted by research and development organizations like the Research Designs and Standards Organization (RDSO) in India. These tests assess the strength, durability, and other performance characteristics of the sleeper.

The prescribed minimum value of 783 is based on the specific requirements and operating conditions of the Indian Railways. These requirements include the type and weight of trains, average speed, and the expected lifespan of the sleeper. The minimum value ensures that the sleeper can withstand the loads and stresses imposed by the railway operations and maintain its structural integrity over the specified lifespan.

Using a sleeper with a lower Composite Sleeper Index can lead to premature failures, increased maintenance costs, and safety hazards. Therefore, it is important to adhere to the prescribed minimum value to ensure the safe and efficient operation of the railway track.