Ok, so I'm trying to finish the game allied with the Railroad (they treat synths like humans and I work on robots so let's just say I'm sympathetic).

Anyway, I've progressed in Underground Undercover, to the point where they say "continue helping Father" because they need time to build weapons. So now I've been doing Institute missions to the point where I went to some round table meeting and they said I would be the next leader.

Than father wanted me to do the Mass Fusion quest, and underground undercover quest said I needed to keep helping father, so I went to relay to Mass Fusion and it said "if you do this you will be permanent enemies with the Brotherhood" . So that gave me pause. It's not that I care necessarily about being permanent enemies with BOS, though I suppose I would rather not. Its just that if I'm going to be permanent enemies with them, I want it to be because I was doing something to help the Railroad!!! Not something that helps the institute!!

So I guess I'm sort of stuck on how to proceed in order to ally with the railroad without helping out the institute too much (or brotherhood). I'm okay with loading an old save, if for instance I need to like never start Mass Fusion or tell Father to f off after Bunker Hill or something like that. Or if you tell me that I have to do Mass Fusion with institute than I will, it just seems that there might be some other way. Is there?

P.S. Please if you reply to help me, don't spoil anything beyond the point I'm at. I have not finished the game with another character. Thank you.

If you want to do the RR questline, you have to do mass fusion which turns the BOS hostile and the Institute questilne up to Powering Up. After that, you get the rest of the RR questline.

However, word of advice is don't do the RR questline. This forum has been literally filled with people making threads about how they hated the RR ending and felt like scum afterwards. Trust me, you'll see why everyone hates the RR and you'll see the short-sighted fanatics with no disregard for human lives that they truly are.

All I'm gonna say is if saving synth is your purpose, you should side with the Institute.

As to your question. You need to do your Institute quest until after a point, so you will need to become enemies with the BoS, you have to do Mass Fusion to continue with the RR.

ok, thanks guys

It's not possible to complete the Railroad quest line without completing Mass Fusion unfortunately. Underground Undercover can't progress until after completing Mass Fusion. I feel like this is one of the more poorly thought out elements of this story line. If you make the Institute hostile toward you prior to completing Underground Undercover, it won't make the Railroad hostile toward you. They'll still be friendly toward you, but you'll be unable to continue their quest line. Completing the BOS quest line requires wiping out the Railroad. There's a bit of an exploit where you can just ignore Captain Kells forever and complete the BOS quest line without completing Tactical Thinking, thus keeping the BOS, RR, and MM all friendly, but it's not really a legitimate ending.

As the title says, you have to work Undercover. This means that you need to finish almost the entire Institute storyline before the Railroad is ready to strike.

It's probably too late to suggest this, but I'd recommend working with the Minutemen, as it allows you to ally with the Railroad, allowing them to continue their mission to rescue synths while allowing things to work out better for the non-synth inhabitants of the Commonwealth.