First, you need to have advanced in the quest "The Hidden City Of Beregar" up until the mission where you need to fix the rails. But If you already got the Dwarven legs from the quest then you're ready to do this part. Getting this achievement is the result of triggering a new mission for the quests and opening a passage between Beregar and Kazordoon which you will be able to use from the moment you do the process on. To start, travel from Yalahar Foreigner quarter to the south west, using Maris ferry to the island of Fenrock, go down the known path until you reach Beregar. Once there, take the stairs left, going towards Emperor Rehal room and take the first ore wagon to the bottom left.
Once inside you will be in the room where you need to order the rails, but this time, you want to order the rails to be able to go from the entrance to the top left path, which you do it like this:
After completing ordering the rails, go back south and take the top wagon, and you will be teleported to another room, in Kazordoon, and the achievement will trigger along with a new quest mission.
Side notes:
-If you walk north you will be able to go through a quest door that takes you to the path to Kazordoon.
-If you go back using the wagon, you will be taken into a new room in Beregar mines which contains a quest door to the north and you can use to get to Beregar.
-The secret shortcut between Beregar and Kazordoon has been an unresolved mistery for years.
-Presumably, there's still quest to unveil from this new mission.