First of all: You have to manipulate the savefiles, so it's not the nicest way.
The condition for the achievement is described a little bit misleadingly, it says "You've completed all DLC maps with 100% progress" which should drop after successfully playing 100% of the Germany DLC (which is the only DLC available right now).
In fact, the achievement didn't drop for me either when I brought all playable maps to 100% in a savefile at the same time (I mean both the 15 standard maps and the 9 Germany maps) although that seems to be the condition.

So if you want the achievement, here's how it worked for me:

1. Disable Steamcloud synchronization in the games settings
2. Find a savefile (preferably the current one), you can find it under C:\Users\user\AppData\LocalLow\Live Motion Games\TrainStationRenovation\Save
(think of the possibly hidden appdata folder, simply enter %appdata% in the Windows search bar, then you are there)
3. If necessary, create a backup of the savefile (if you are unexperienced with savefile editing).
4. Open with text editor and search for levelPercentFinish (search direction downwards)
This should now be found once for each of the 24 levels, followed by the number that belongs to the completion of the level.
5. Now you set each of the 24 numbers behind levelPercentFinish to 100, strangely enough, some of mine were back to 0 after playing the DLC.
6. Save file and close
7. Start the game and load the savefile, then the achievement should drop 😊


Let me know if this helped you or ask if you don't understand something. Of course, this way is also a way to cheat many other achievements (e.g. the master achievements of the DLC). Do what you think, I don't judge.