I will not give hints for conversations, that is entirely up to you.
Once you get to the railroad crossing, talk to Ben.
EDIT: I've noticed that since this chapter and the next take place in the same place, it's possible to skip the next step and go inside the locomotive immediately. You'll have to get the water bottle and railroad map eventually though.
Enter the wagon, and you'll notice a recently used 'camp' inside. In it are a water bottle and a railroad map, take both.
Open the door to the front of the train. Walk on the locomotive, to the machinist's cabin. Look through the door and you'll see a walker. Open the door and dispose of the (non-)walker. Investigate the body.
After Ben de-activates the braking system, talk to Kenny and the achievement should unlock.