I've been exploring this map for a while now and found some secret stuff. I couldn't find out how to destory the core though.
So far I´ve found out:
1) There is a secret train that spawns in station 606 when you bring the bottle with 606 written on it, that you can find in the "Food Mart" in the small town, to the "Library" and put it in one of the wooden locker things.
2) There are 2 power sockets in front of the abandoned station that leads to the core. The "secret train" has a plug so they probably have something to do with each other.
3) There is a "Train memorial" room in the top level of the map. Just follow the rails at the spawn behind the train and you might end up in some kind of black and white tunnel. You can´t really get in there cause it will teleport you out. Just take a prop and "drive" it in there. You will find some facts about the map and a code: 141568.
4) There are at least 6 buttons below the "override button" in the core room. I guess that only these buttons can be used to open the door but i didn´t find out how to do it yet.
[5) The code for the service room is 2419.]
So if you find out what the code is for and how to make the core explode feel free to write it in the comments. I´d really appreciate it.
EDIT: Okay I just found the facts.txt file that was mentioned in the "Train Memorial". It contains some really interesting stuff, that I´m not gonna write here. To find the text you need to open the map file with an editor, press ctrl+f and search for "facts.txt". But the way I see it it contains "bad" news.