Railsign

Railsign is a secret marking system used by the Railroad in Fallout 4. Each marking consists of an "asterisk" of six or eight white lines, at the center of which is a special symbol. A visual chart of the railsigns appears in Railroad HQ.

Locations

 * The railsigns appear in the quest Tradecraft, pointed out by Deacon, being used as indicators to Ricky Dalton who is referred to as "The Tourist."
 * Railsigns can also be found across the Commonwealth, albeit rarely. Most are found at Railroad quest locations, such as safehouses, dead drops, and around the Switchboard.

Behind the scenes

 * Railsigns are similar to the Thieves Guild shadowmarks from Bethesda's The Elder Scrolls V: Skyrim. Both are glyphs used by underground societies to designate certain places and things to other members of the society.
 * They also bear a similarity to the real life hobo code - symbols used by the homeless to let others know about various things related to their lifestyle, such are safe places to camp or places to get food.