curve. sunday's accident has renewed calls for positive train control or ptc, safety technology that can reduce train speeds based on real-time track conditions. >> fully operational train control system could have avoided this accident. that's what it's designed to do. >> originally supposed to be fully installed by the end of 2015, congress extended the deadline for implementation to the end of this year with the possibility of further delays. >> it is so important in this country that ptc be implemented by the end of the year and that we don't give any more extensions. >> the ntsb says track signals of the crash site were not operating at the time because they were undergoing upgrades for a ptc system. investigators plan to remain unseen through the weekend and then the ntsb will issue a preliminary crash report a few weeks after. >> bret: jonathan serrie live