As you progress through the mission, you'll come through a post office and get into a truck to chase the train.
The goal is complete this 1 minute driving section without hitting anything. Watch out for rocks, bushes, trees, bridges, sidewalls, and fences. Your tires should be touching the ground, and that's it! Complete this entire section without touching anything in order to unlock this. If you complete it and don't get the achievement, you'll have to restart the entire mission as it ends with a checkpoint.
If at any point you think you hit an obstacle, immediately pause the game and choose Last Checkpoint