If you ask Counter-Strike players, many of them will tell tales of a map called de_train. It has become a classic map, thanks to its layout and its visual characteristics. Alas, the old Train has aged over the years, but Valve recognizes the love this map has received, so they're re-constructing it from scratch for Counter-Strike: Global Offensive.
Valve lays out everything players need to know about Train, including some changes to each bombsite. Some of the layouts have been tinkered with to ensure a more strategic match and allow players to more easily search certain hiding spots. The middle tunnel has also been removed entirely. The second Bombsite has received similar tweaks, which you can read about on the Counter-Strike blog.