OUTDATED

Thanks to Tadness for helping me with this. Make sure you turn on speedrun mode by pressing L during the loading screen so that there is an in-game timer.

The beginning of the run using a technique known as "buffering" or "lag clipping", where you freeze the game window to freeze your character in place. When the G-Man says "Wake up and... smell the ashes," start holding the left-mouse button (click) on the top of your screen. Let go just after you hear the train horn. If performed correctly, You should end up outside of the train.

After "escaping" the train, you should see some boxes to your right. Make your way over to them and pick one up. Then, crouch and look down so that the box appears inside of you. Let go of the box, and the hitboxes should be merged.

This part requires a bit of luck. You and the box will fly upwards and (hopefully) sideways, putting you on the roof of the station. Once you're close to the top, re-pickup the box and throw it away from you so you don't get stuck flying forever. You'll want to navigate your way towards the back, looking for a hallway that juts out as shown here:

After you get here, drop down the backside to land on the carpeted area below. Press yourself into the corner shown here, and the timer should stop. You can also trigger the timer's end point from the top of the corner without having to drop down.