I have no idea if this has been discussed yet. If it has I apologise, I haven't been on in a few days after being extremely active for the first few days.
I'd like to drastically point out all the signs I have seen with the link of the Zombies map "Tranzit" and the Multiplayer map "Express.
One of the similarities I have noticed with this is that it is Located in Los Angeles, in a bullet TRAIN centre. We all know that there is rumours of a train in Zombies, yes? Good long to medium engagements inside the Los Angeles bullet train center. is the map description.
The other possible connection I noticed was that the train has the words "Transit" on it. Of course it is spelled with an S instead of a Z, but it is still possible. I don't think they would've just named the train line something with 'Transit' in it without any of us getting suspicious.
It is located apparently in LA, which is near Nevada (possible link to Nuketown Zombies)and it is also very close to Hanford, CA. This is a very plausible link and it could be the answer to those train sounds we have been hearing.
What do you guys think of this? Let me know if there are any flaws in my explanation and feel very free to prove me wrong (Tac :roll:).