U.S. railroads may not be obligated under federal law to carry freight including crude oil and hazardous materials from Jan. 1 if they fail to meet a year-end deadline for implementing new train safety technology, according to a top federal regulator. The letter, reviewed by Reuters, presents the most tangible sign yet of what could lie ahead for rail carriers and their customers, if Congress fails to extend its Dec. 31 deadline for railroads to implement positive train control, or PTC.