Let the length of the bridge be x meters.

Running towards the train, Chetan covers 0.5x-10 meters in the time that the train travels x-4 meters. Running away from the train, Chetan covers 0.5x+2 meters in the time that the train travels 2x-8 meters.

Because their speeds are constant, the following holds:

(0.5x-10) / (x-4) = (0.5x+2) / (2x-8)

which can be rewritten to

0.5x2 - 24x + 88 = 0

Using the quadratic formula we find that x=44, so the railway-bridge has a length of 44 meters.