It lies just south of Monsignor Plaza and can be accessed from the exterior either via an elevator on the first floor or by a door which is up a couple of flights of stairs. The door, however, can only be unlocked by using a terminal located on the bottom floor of the safehouse interior.
The safehouse has eight floors, two above where the elevator drops the player character off, and five below. Each floor is relatively small, but added together there's quite a lot of space. The Railroad only occupy the top three floors, which appear to have been renovated and cleaned, compared to the bottom floors.
Ticonderoga and its map marker are only accessible after completing the quest Boston After Dark for the Railroad. If the player character sides against the Railroad before completing Boston After Dark, neither the main door nor the elevator will be usable, permanently barring the player character from the base for the rest of the game.
On the first floor the elevator drops the player character off on, there is a fridge near the Fat Man. It has a bunch of random loot and a cooler, however there is no prompt to open/access any of the content inside of the fridge.
There is a protectron locked in a pod in Ticonderoga, and it is capable of being Legendary. If it is Legendary and the player character manages to destroy it, the mini-nuclear explosion may kill some of the Railroad agents standing on the floor above it.
Even if the player character allies themself with the Railroad faction, there is a locked terminal on the main floor that they cannot access without it being considered a crime.