1. Longest rail tunnel in the world ?

Answer: Seikan Tunnel (Japan - 53.9 Km)

2. Which is the Densest populated State of India ?

Answer: West Bengal

3. Which is longest tributary river of India ?

Answer: Yamuna

4. Which is the longest Platform ?

Answer:

Gorakhpur railway station, Uttar Pradesh, India (1355.40 meters)

5. Which is the Densest populated State of India ?

Answer: West Bengal

6. Which is largest Delta of India ?

Answer: Sundarban (W. Bengal)

7. Which is highest dam in world ?

Answer:

Jinping-I Dam - 305 m (1,001 ft) in China on Yalong river.
Second Highest is Nurek Dam 300 m (980 ft) Tajikistan on Vakhsh river.