This achievement is not very difficult, but it is time consuming and will take around 8 - 10 hours to complete.
If you have already completed the scenarios, you should have about 15% progress to this achievement already.
Go into the Services menu and select the Class 66 DBS train. Continue through to the list of services. We wish to choose a service that covers a large distance but has few stops. I personally chose the 04:32 4M21 Grain Thamesport to Brierly Hill service, as this covers 31 miles*, has only one stop and has no red signals. Begin the service.
Once in the game, go into the settings menu and ensure that you have 'Disable Junction Derail' set to On
- this allows you to pass through junctions at any speed without failing the scenario/service, effectively putting the game into easy mode. However, you can still fail by passing through a red signal or by passing the final stop of the line at speed.
When the service starts, jump out of the driver's seat and exit the cab using the two doors behind you. Head to the back of the locomotive and uncouple the cargo carriages (this process is covered in the 4th scenario, Aggregate Industries). Return to the cab and get back in the drivers seat. Now, press forward on
to put the reverser in forward and hold
to throttle up. Now, let the train accelerate and continue on its journey. This takes about 35 minutes, so have some other entertainment on hand while you wait.
The train will quickly reach a speed of 75 mph, also unlocking the Pushing the Limits achievement.
At the end of the journey, let the train pass through Reading station and fail the service (this will not award any XP). You now have the option to restart the service or return to menu. Keep restarting as many times as you need in order to unlock the achievement.
One complete playthrough will award you with 7% progress* - starting from scratch, that is 15 playthroughs. As each full playthrough takes around 35 minutes, that is around 9 hours of gameplay to unlock this achievement.
Once complete, sit back and enjoy your 100G achievement - you definitely earned it!
*Each run actually gives slightly less than 7% progress. Also, despite the objective marker when you start the service showing 31 miles, 7% of 660 miles is actually around 45 miles.