This is a closed-off area in College Square that was formerly a train station that was converted to a raider hideout until after it was overrun by ghouls. The area itself is not that large and is also part of the quests "Ghoul Problem at Tenpines Bluff" and "Cleansing the Commonwealth"
There is a first aid kit container and some meds behind the shops and the restrooms. There are two sets of stairs leading to the eastern and western tunnel. The eastern tunnel doesn't have anything notable except for the two Nuka-Cola Quantums found in the Nuka-Cola Machine near the stairs. The tunnel to the west has a Novice terminal where you can activate a protectron upon a succesful hack.
The ticket counter in the upper floor has an Expert lock. You can get the College Station Key from the Maintenance room in the western side of the station. Be careful since there's there's a Feral Ghoul Stalker inside. The key is found on a table in the room. Alternatively, there's a terminal in the western wing as well that will activate the security protectrons in the station. The protectron inside the subway token booth will walk out of it and will allow you to enter it.
The subway token booth has several pre-war money inside. The notable item there is the Live and Love magazine which will increase your companions' damage by 5%.