Once you make it to the front of the train you must choose who to leave there and which character to go to the back. I chose to leave Rebecca and go out as Billy.
Pick up the black key card and ammo, exit the door and run past the two Umbrella Corp Zombies and exit the through the door. Go through the conductor room and up the ladder. Pick up any items you left behind after your encounter with the Stinger.
Continue down the stairs, kill the STARS member (takes more rounds than the other Zombies) and move to the back of the train.
On the left, slide the black key card and input one of the following combinations:
36 - 3333333339
67 - 7777777774
81 - 8888888889
The number to the right will let you know which combination to use. It will then cut back to Rebecca. Check the dashboard in front of you, insert the combination and whammy!!! Cut scene starts, trophy pops, and the train is complete.