There is more to this than that story shows.

Of course the girl has a responsibility and at first glance it's difficult to see how the guard can be blamed. The reason why (you may still disagree with the conviction) is that the guard was fully aware that the young girl was drunk and that she was leaning on the train. Knowing this he signalled for the train to move away. The movement of the train caused her to fall between the gap.
Yes she was drunk, but that doesn't remove his duty of care as the train guard. She was knocking on the glass to get back on as she had mistakenly gotten off. He assumed that although she was leaning on the train she would move away if it started to move off. He was wrong in that assumption and it cost her her life. I think that qualifies as gross negligence. And gross negligence that results in death is manslaughter.

Also, if people weren't held to account for mistakes or bad practices in work we would still be in the days of scores of men being chewed up in factory machinery with no accountability.