The correct answer is (4) 50 metres

Solution

Let the length of train be x km

Let the speed of the train be y km/hr

2 kmph = (2 x 5/18) m/sec = 5/9 m/sec.

4 kmph = (4 x 5/18) m/sec = 10/9 m/sec.

According to the given condition

 x/(y-2) = 9/(60*60)
x/(y-4) = 10/(60*60)

On combining both the equations we get
9y – 3600x = 18
and 10y – 3600x = 40
y-400x = 2

and y – 360x = 4
Therefore
40x = 2 

x = (2/40 *1000)m

= 50 metres