You'll get this while the train is moving.
*** Spoiler - click to reveal ***While speaking to Katjaa, you will realise that Duck is almost dead as a result of his bite.
You need to go and get Kenny to stop the train.
It is possible to talk Kenny into stopping the train. I showed him the bloody tissue first and then tried to pick the most suportive parts of the conversation that follows.
If you chose a different conversation strand, you end up in a fight and basically beat him into submission.
When the train begins stopping, you get the achievement.