"After a good year of hype that wasn't supposed to exist, we finally got our hands on Half-Life 2 recently, so we'll take you through it chunk by chunk.
You start off the demo on a train. You can walk up to a couple other passengers and they'll talk to you, saying things like "I didn't see you there," as if you appeared from out of nowhere. After a half-minute or so, the train comes to a stop and the door opens so you can get out. This is the end point for the train tracks, so you can't stay on and go to the next stop or anything like that. We're not certain exactly how this portion will match up to the beginning of the final game -- this 30 second or so portion we saw could be only the very end of a long train ride to start the game -- but it is notable that the part Valve showed us was very quick and a lot more to the point than the long introductory sequence of the original Half-Life."
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