The Metropolitan Catacombs were a section of Metropolitan City built beneath the grounds as a network of series of cave-like tunnels & sometimes tubes that cannot be traversed on foot but rather by a special vehicle.
The caves are sections that take most of the city's underground area. They are usually traversed by foot & look similar to basements. They are where the Catacombs are usually entered or exited.
The subrails are long circular tunnels that cannot be traversed on foot, but by the Catacomb Transport. These railways are able to cut long journeys through the Catacombs short as they can link one section of the caves to the other, while having the transport go at breakneck speed. However, the subrails are full of machinery that can cause death to those unprepared to face them. It's most likely this machinery was installed by the Imperials some time after they learned of the Catacombs.
First Dark WarEdit
The catacombs were built first in the Philadelphia Ruins when the Global State-Alliance for the Survival of Humanity busily prepared the creation of Metropolitan in an effort to find a haven for the Alliance that would draw the shadowghosts away from New York II.
Second Dark WarEdit
When James Anderson visited Metropolitan City, he discovered that there were two entrances to the Catacombs from his section of the city: one in the very end of the backyard alleyway & one somewhere concealed his cousin Maureen's basement. Upon taking the entrance in the alleyway to a subrail that led him to another end, he came out on the Mine Bridge. However, this exit was destroyed when the Mine Bridge collapsed in the ensuing skirmish. James then took a trek through the Catacombs once more through his cousin's basement. After riding the subrails, he found another entrance that led to the Metropolitan Cathedral in Logan Square, where the Alliance was. To assure that the Imperials couldn't get through this way, James destroyed the entrance. Another entrance was found at the very bottom of the Academic Military Research Institute's Time Tower & this way led to the main fortress of the Imperials.