Is the only way to get this to join the railroad?

yeah

However you get it relativly quick by doing stuff for them so you are not locked into Railroad faction if you just want to get the ability to make balistic weawe.

Be mindful of some of the quests though. I started the Railroad side quests, got the first one where you have to escort a synth to a safe house from the Doc. BUT that quest bugged out on me, the synth didn't show up after I cleared the area. It's been a month (in-game), I check back once a week and still nothing. I've done all the weathervane missions but I can't progress until I finish that escort quest. PAM won't offer the special quest until I progress further.

Loading an older save file isn't optional for me. I've done far too much now to revert.....

So be careful when doing Railroad side quest, especially the one the Doc gives ya to save the synth slave. Make sure to make a save before embarking just in case it bugs out on you too.