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

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

Let the length of the train be x metres and its speed by y m/sec.

Then,		x		= 9 and		x		= 10.
		y - 5 9				y - 10 9

 9y - 5 = x and 10(9y - 10) = 9x

 9y - x = 5 and 90y - 9x = 100.

On solving, we get: x = 50.

 Length of the train is 50 m.