Metropolitan Transportation Authority's two commuter railroads Metro-North and the Long Island Railroad (LIRR) reached the Positive Train Control (PTC) milestone eight days ahead of a federally mandated deadline to have the safety overlay system functioning on all required lines and locomotives. The rail safety system was required to be installed by Dec. 31, 2020. Positive Train Control is the ultimate in modern signaling and train control, its widespread adoption on railroad mainlines carrying a large number of passenger trains or dangerous freight was mandated by the US Congress in the Rail Safety Improvement Act of 2008 (RSIA) after a terrible commuter-freight train head on collision in 2008 at Chatsworth, California. The four primary functions of PTC are: train separation/collision avoidance; line speed enforcement; temporary speed restrictions; and rail worker wayside safety. PTC can be a standalone system that completely replaces older signaling systems, but in the USA its primarily being installed as an overlay of the existing signaling system.