The easiest way to get this I found was to go to Timetable/Services and pick any class of train. I chose the 66 DBS. Select any destination. I chose 6V04.
When the level starts, press Y to get out of the chair, turn around and open the door behind you with A. Then open the door on the right with A.
Hope out onto the tracks, pick a direction you want to go, hold down RT (sprint), and start moving down the track. After 1 mile you will get some reward levels and your achievement.
I do not know if running on the track (causing the red crosshairs) counts. To be safe, run in the middle of the tracks so your crosshairs are a small white ball.