1. Go to Timetables and Spawn Walking at Gemünden at 13:15.
2. Wait a short while and a train will pull into the station at 13:17 a couple platforms to the southwest of where you spawned.
3. Get into that train as soon as it stops and go to the driver's seat and press
to take over
4. Confirm that you want to take over the service
5. Wait for the Unload Passengers circle to fill about 75% of the way and pause and Save
6. Let the ring fill the rest of the way and close the doors and finish the service and quit to the main menu after.
7. Select Last Played from the main menu and keep repeating steps 6 and 7 until the achievement pops.
The tracker does not work for this achievement so you will have to keep track manually if you want to know how many you've done and how close you are to 100.