I already killed all of the people in the railroad as a BOS quest, and my armor is trash.
According to the Wiki for this crafting mod:
This crafting ability can be learned from Tinker Tom after performing certain quests for the Railroad faction. Once this skill is learned, one can upgrade any applicable clothing item at an armor workbench.
After completing the first or second jackpot mission, Drummer Boy will tell the Sole Survivor that they need to speak to Tinker Tom, who will inform them about the ballistic fiber technology found in the DIA cache. At this point the Ballistic weave is unlocked at any armor workbench and Tom will sell some armored gear of his own.
Since you've taken out the Railroad already, and haven't learned the recipe prior to doing so, you won't be able to learn it anymore. Only Tinker Tom can teach it to you. Even if he was still alive, he'd likely be hostile towards you since you've killed other members of the Railroad.