Just do their quests and you'll get it. You don't need a specific armorer rank to use it just to upgrade it and you can upgrade it at each rank with the best obviously being at full armorer.
of If your on the pc, enter the following into your console command window "set railroadclothingarmormodavailable to 1" and this gives you the perk. I did this because my character refused to join the RR.