The speed of the train is 81 KMPH.
The calculations are as follows:
Distance travelled by first pedestrian in 8.4 seconds at 4.5 kmph = 4500*8.4/3600 = 10.5 m
Distance travelled by second pedestrian in 8.5 seconds at 5.4 kmph = 5400*8.5/3600 = 12.75 m
If L is the Length of the train, the train would have to travel L+10.5 meters to fully pass the first pedestrian and L + 12.75 metres to fully overtake the second pedestrian.
If S is the speed of the train, S in meters per second = (L+10.5)/8.4 = (L+12.75)/8.5
Solving the above equation, 0.1L = 17.85. and L = 178.5 meters.
If it takes 8.4 seconds for the train to travel 178.5+10.5 meters, the speed is: S = Distance / time = 22.5 meters per second, which is equal to 22.5*3600/1000 = 81 KMPH.