Cecilia didn't even need to be told by Charlotte, as soon as she saw the men in the truck she sprung into action. She ran to one of the unoccupied seats in the train, pulling out her her main rifle. These people were a waste of her time, and because of that, they weren't going to get out of this. Cecilia would make sure that these men would't survive their little stunt, 

Quickly, she aimed her weapon out the side of the window, directly at the man with the RPG's head, making sure she couldn't be noticed. From this angle, they couldn't see her, but she could easily aim and see them, For most people, a bullet shot out of a moving train trying to hit a moving truck from this angle would be difficult, but Cecilia was better than the rest. She couldn't miss this shot if she was blind.

Cecilia readied herself, and pulled the trigger, waiting for the results. Odds are, the man wouldn't be able to dodge the shot, and he would be in hell within the next few seconds. Taking out the others would be much easier without the man with the RPG.