This achievement will unlock at the end of chapter five of episode three
*** Spoiler - click to reveal ***After getting the train working (use the pencil in the RV to get an outline of the train instructions, then use the monkey wrench to uncouple the car) you will have to either convince or fight Kenny to stop the train. Once the train has stopped, the achievement will unlock.
There have been some delays and glitches with some of the achievements in this game. The best way to fix this is to just continue playing and wait for them to unlock.