Answer
$0.20\ km$
Work Step by Step
Given radius of the track $r=3.5\ km$ and the subtend angle $\theta=3^\circ20'=(\frac{10}{3})^\circ$, the length of the train, which equals to the arc length, can be found as $L=r\theta=3.5\times(\frac{10}{3})^\circ\times\frac{\pi}{180^\circ}\approx0.20\ km$
