(That isn't as blunt an instrument as it sounds. If the relay is activiated, it can switch +12v DC to the track after the signal. If the train decoders are programmed for "brake on DC" then the train will come to a smooth stop.)
To use this function, a DTX8B add-on board will be needed. The outputs of that board, instead of driving additional signal masts, will control one relay for each of the 8 signals driven by the board. The board can't directly drive the relay coil: an additional driver will be needed. We supply the RLY2: a dual pole relay with transistor driver assembled onto a circuit board.
This document gives a circuit diagram.
Note: The circuit diagram and information in the attached PDF shows "CML Electronics Limited". This should now be read as "Sig-naTrak® by GFB Designs" at the address details on our Contact Us page.