Answer
$ a>4.45ms^{-2}$
Work Step by Step
time taken by train to reach crossing $= \frac{distance}{speed} =\frac{60}{30} =2s$
Thus writer must have taken less than $2s$ to cross the crossing.
let a be his acceleration
distance travelled before he applied brakes $= vt = 20*0.5 =10m$
now he is $35m$ away from crossing
remaing time for him to cross the crossing is $2-0.5 = 1.5s$
$\therefore s< ut +\frac{1}{2} at^2$
$ 35< 20*1.5 +\frac{1}{2} a (1.5)^2$
$ 70 < 60 + 2.25a$
$ a>4.45ms^{-2}$
