To earn this achievement you are required to travel 380 miles in the GP38-2.
A good service to run for this achievement is the "F04-2 Train train for Fresh Pond Junction" service which is 8 miles, but can be up to 10/11 by not completing the scenario - progressing your tracker by 2/3% each time. Each run takes anywhere between 20/40 minutes depending on how active you are with changing your speed to follow the limits.
I've been leaving my brakes in the positions that they start at and setting the throttle to 8, dropping it slightly whenever I hit a lower limit, Leaving the brakes on is completely optional (but I've been using it as an opportunity to catch up on netflix).
It is important to note that you must be at or below the speed limit, if you are above progress will not track.
Credit to: Alf Ganikiller for confirming this.