You'll get this while working to clear the tanker from blocking the train. After meeting Omid and Christa, you go to check out a station. Clementine will come with you.
First check the door round the side and lift Clementine up to it so she can look around. She will unlock the door.
Once inside you need to prop the door open with something. Use the tool you got from uncoupling the train earlier in the game.
You need to pass Clementine over the top of the bars so she can get the keys. When she is in there, you will be attacked by a couple of walkers. Move back to the tool wedging the door open and deal with them.
You then notice that there is a walker in with Clementine. Quickly grab the keys from her and open the door. Then shoot the walker. Grab the blow torch and head back to the train.
Go up to the brigde with Omid. Try using the blow torch on the trailer coupling. The blow torch has a leak. Use some tape in the rear of the armoured car to the left of the screen to fix the hose. The use the blow torch again.
To finish cutting the trailer loose, you will need to dangle Omid over the edge.
You should get the achievement once the trailer is moved and Ben notices what is heading your way...