Signal Link is used to link a Slave signal with a Master signal. Both signals must be named and both must be able to show the same aspects. Add the Signal Link box in the Route Layer. Configure the Signal Link box in the Session Layer.
If a tick is placed in the box next to Clone danger state only, then if the Master signal shows a red aspect then the Slave signal also shows a red aspect, but if the Master is not red then the Slave signal shows its normal aspects.
Without a tick in the box, the Slave signal will replicate the aspects shown by the Master signal.
Tested with both Master and Slave as Signal USA 04. The material remains copyright by JointedRail. Users must respect the license conditions. All I have done is update it for use in TS12 and uploaded it to the Download Station, with permission.
One application, to control access by trains to a single line section of track, can be seen here
http://forums.auran.com/trainz/showthread.php?t=83432