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.

Appearances
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 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.