Train Sim World 2020

Warning ahead: VERY grindy achievement.

First of all plug your controller into your xbox, than grab a second device and binge watch a few seasons of HIMYM or Scrubs or whatever you like.
If you have a second console - play something other there on an alt account.

1. Open Route RSN (Ruhr-Sieg-Nord)
2. Go to Timetable
3. Select the BR155
4. Take the first job DB81101 (or if you want to drive by daylight: DB81103)
5. Detach the cargo from your loc, hop in and prepare everything (don't forget the brake key!), set your speed to 4
6. Drive until you reach a 120kph/74mph zone
7. Make a Save here
8. Stay on speed 4 and do not touch your controller
9. At the End stop at the marker or drive through, doesn't matter
10. Reload your Save
11. Repeat step 8 to 10 for a total of 30 times.

You will likely get ~50,6km (~31,4mi) per run in about 42 minutes.
This will take you overall 31 runs which equals to about 21 hours.

With speed 4 you don't have to worry about getting in trouble with the speed limits as those are between 70 and 120kp/h (~43.5mph and 74.5mhp) and you will get the most out of it.

With that being said: Happy grinding!