SelTrac is a digital railway signalling technology used to automatically control the movements of rail vehicles. It was the first fully automatic moving-block signalling system to be commercially implemented. SelTrac was primarily sold and developed by Alcatel, through a subsidiary. SelTrac is now sold by Thales from their Canadian unit, after it purchased many of Alcatel's non-telephony assets. New versions were made for different markets, and today SelTrac is used for rolling stock control around the world and is branded as a communications-based train control (CBTC) system.