The train was in motion when he stepped out. The inertia wanted him to continue in its present state of motion. He could have counteracted it by running as soon as he stepped on the ground and slowly stop.

Hahhahaha. This has happened to me before. But it was a bicycle. It was then I learnt that when you jump down from a bicycle or moving vehicle, dont stop, keep running till you can gain the stability. This post is now classroom based. Weldone @greenrun