Keys are always located in the same place, in the passage leading to the generator room on a desk.
The switch to start the generator is one of three throw switches, the only one not thrown.
Wait in the cargo room. Make sure that the shutter has been closed since it allows zombies to crawl into the cargo room directly from their spawn point.
Once the first generator fires up, a shutter will open in the cargo room. The secondary generator is located through this shutter.
Two or three survivors should break the boards leading to the secondary generator. The button can be found in the room covered by the third set of boards.
The rest of the team should watch the cargo room stairs, as they are the only point of entry for zombies.
When the secondary generator fires up, the doors on the bottom of the cargo room will open. Go through these doors.
WARNING: Standing on the doors when they open can result in death.
Once through the doors, move away from the hallway nearest to you. You will reach Platform 2, which has a door covered by boards.
There is always a wood plank located near the wooden box by the security room. It is one of the best melee weapons for breaking boards, so use it!
Break the boards on the door to gain entry to the security room. The button is large, red, and circular, and can be found near the computer monitors.
Start the train
There is a red button in the generator room. Use it to start the train.
Camp on the train or in the rafters, or optimally, both. Watch the doorway near the train especially, since zombies spawn down a ladder from the train.
If you didn't get aboard the train in time, the transport tunnel in the room next to the desk where the keys appear opens approx. 2 minutes after the train leaves. The end behind the construction lights is the one that opens.