Scram is an achievement/trophy in Metro: Last Light which is awarded to the player for keeping all of the watchmen from boarding the player's railcar in the level Regina. If a watchman jumps on Regina, the achievement/trophy is void and the player must restart the checkpoint.
To do this easily, it is advised to play this level in Hardcore or Ranger Hardcore difficulties as the enemies take a lot less damage before they die, meaning it will be easier to kill them (although you will be easier to kill too). The best strategy is to be skilled in quickly swapping between an assault rifle for long range killing, and a shotgun for when they get close. Both a heavily modified Kalash and a Shambler can be earned during the course of the level, making the pursuit easier for unequiped players. It is also a good idea to use military grade rounds in the Kalash, granting even more damage in each shot - if MGRs are used, players should keep in mind that there is no penalty for re-loading the checkpoint to get the rounds back, once the achievement/trophy is earned.