Which of the following types of sleepers has the best shock absorbing capacity as well as damping property?

 1. CST Sleeper (Composite Sleeper):  
   - Made from composite materials (e.g., recycled plastics, fiber-reinforced polymers).  
   - Offers moderate shock absorption and durability but lacks the natural elasticity of wood.  
   - Damping properties depend on composite design but generally inferior to wood.  

2. Steel Sleeper:  
   - Highly rigid and durable, ideal for heavy loads and high-speed tracks.  
   - Poor shock absorption due to stiffness; vibrations are transmitted rather than absorbed.  
   - Minimal damping properties as steel lacks internal friction to dissipate energy.  

3. Concrete Sleeper:  
   - Extremely strong and long-lasting, widely used in modern railways.  
   - Very rigid, leading to poor shock absorption; vibrations transfer directly to the track bed.  
   - Low damping capacity due to brittle nature and lack of flexibility.  

4. Wooden Sleeper:  
   - Natural elasticity allows it to flex under load, absorbing shocks effectively.  
   - High damping properties due to internal friction within its fibrous structure, dissipating energy as heat.  
   - Historically preferred for noise reduction and vibration damping, though less durable than steel/concrete.  

Key Reason: Wooden sleepers excel in shock absorption and damping due to their organic, flexible structure, which deforms slightly under stress and dissipates energy naturally. Modern materials like steel and concrete prioritize strength and longevity over these dynamic properties.