The Railroad are essentially smugglers. Their mission is to smuggle slaves to safer areas and avoid detection. They simply believe synths should be treated the same as regular humans. They used to help human slaves as well according to Des but now focus only on synths because no one else would help them.
I don't know why you think they murder innocents since that's never supported in the game. And the fact you can't imagine the possibility of a synthetic human just shows your imagination has some limitations for whatever reason.
So the Railroad will never be a group you understand because you need to be able to imagine that an synthetic being could be considered a person.