WASHINGTON (AP) — Congress passed a bill Wednesday that delays for at least three years the mandate for railroads to put long-sought safety technology in place, and extends the government's authority to spend money on transportation programs. Railroads have already had seven years to install PTC, but most aren't expected to meet the end-of-year deadline to put it into operation on all tracks that carry passenger trains or are used to haul liquids that turn into toxic gas if spilled. Positive train control relies on GPS, radio signals and computers to monitor a train's position and automatically slow or stop trains that are in danger of colliding or derailing due to excessive speed. Freight railroads say they will stop hauling cargo like chlorine, which becomes a toxic gas when exposed to the air, and prevent commuter trains and Amtrak from using their tracks if the deadline isn't delayed. The law mandating positive train control was passed in response to a 2008 collision between a Metrolink commuter train and a Union Pacific freight train in Chatsworth, California, that killed 25 people and injured 102 more.