In velocity of the train `u=100 ms s^(-1)`
Final velocity of the train, `v=0 m s^(-1)`
Time taken by the ratio to come to rest, `t=5s`
`:.` Acceleration of the train, `a=(v-u)/(t)=(0.100)/(5)=-20 ms^(-2)`
Hence, retradatio of the train `=20 m s^(-2)`