This is a missable achievement.
Once out of the tunnel drive and shoot your way trough the mission in your tank. You will naturally come across the train station but i also think that a secondary objective was there. Just kill all the enemies there so your allies can land and the achievement should pop.
Edit (thanks to Typhojd for finding out):
It seems that you can actually miss this achievement if you just drive through the station. To prevent yourself from playing this mission again or reloading the last checkpoint make sure to kill ALL of the enemies. Use the tank and shoot everything there (also shoot to the hills on your right as the korean troops will get some reinforcement from there).
To be sure that you´ve done everything right just not leave the station until your achievement has unlocked.