This John Plocher design, the HO DCC Flag man, gives the Time Table and Train Order (TT&TO) operator a convenient way to "flag out" and warn other trains that he's occupying main track, it also serves as handy DCC track power tester!  The Flag man has contacts that bridge the track (and trigger occupancy circuits) and light an LED "lantern" to warn approaching trains of his presence.  The LED Lantern may be optionally placed high or low.  The 4-pack part is a revised design by John Plocher and more information may be found here.  You will need to solder a resistor and LED to the Flag man's body and then solder the flag man to his base.  Weights are supplied. 

Assembly Instructions

Instructions for Blue Flagman

Here's a link to nice video by Miles Hale on how to assemble the flag men

Also available in an 8 pack