Railsign

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

Symbols

 * Plus (+) - Ally - denotes an ally is present nearby.
 * Square - Cache - a supply cache can be found nearby.
 * X - Danger - warns that danger is present nearby.
 * Arrow - Pointer - shows the direction to something important.
 * House - Safehouse - a Railroad safehouse is nearby.
 * Teardrop - Dead drop - a Railroad dead drop is nearby.

Behind the scenes
The railsigns are similar to the shadowmarks from The Elder Scrolls V: Skyrim, the latest entry in another video game series developed and published by Bethesda. Both are glyphs used by underground societies to designate certain places and things to other members of the society.