The Railroad ending and Institute ending both destroy the Brotherhood of Steel, so it can’t be those. Fallout essentially had to choose one of the endings here in order to keep the story going.

Some of Fallout 4's endings are better than others ... but she knows a group that does: the Railroad. If you have completed the "Road to Freedom" quest and have already found the Railroad ...

However, given Fallout’s post-apocalyptic setting and ... Deacon, a member of the Railroad, claims that Tinker Tom restored the gun and that it is one-of-a-kind due to its combined firepower ...