Hard to tell from this image, but it makes a lot of sense. The Minutemen survive the events of Fallout 4 in all endings, and since the Sole Survivor is their general and the Brotherhood is obviously also still around, it's likely they formed an alliance.

That begs the question though, what happened to the Railroad? I mean, The Institute is fucked if the Brotherhood’s still kicking, but if the Minutemen won(probably not) then the Railroad survives as well. But honestly, I hope they do something different altogether: a loose unity. Think about it: The Institute would be the scientific branch, the Railroad would be the espionage branch, the BOS would be the military branch, and the Minutemen would be the civilian relations branch. But, if we can’t have that, I’d like the Boomers to join the BOS

Backlight prevents a clear view, but as previously mentioned, RR is espionage. Optionally, they can have a 'heavy' agent who also happens to be the Minuteman General, and possibly even a high-ranking soldier in the BoS. Either they continue to covertly feed important intel to the key agent, and/or they take on a roll as a new scientific division of the MM (if the Institute is eliminated).

[This thought is just based on FO4 play, not the show itself.]