The total distance covered by the train is 15km + 75km + 10km = 100km.
The time taken to travel the first 15km at 30km/hr is 15/30 = 0.5 hours.
The time taken to travel the next 75km at 50km/hr is 75/50 = 1.5 hours.
The time taken to travel the last 10km at 20km/hr is 10/20 = 0.5 hours.
The total time taken for the entire journey is 0.5 + 1.5 + 0.5 = 2.5 hours.
The average speed for the entire journey is the total distance covered divided by the total time taken, which is 100/2.5 = 40km/hr.
Therefore, the average speed for the entire train journey is 40km/hr.