How would New York City respond if a bomb filled with arsenic trichloride, a highly toxic liquid compound, were to explode on a freight train moving through a Queens railyard — just when a commuter train carrying weekend passengers was traveling in the other direction? Emergency workers would rush in, try to determine what threat was involved and begin decontaminating the dozens of wounded. A temporary morgue would be set up to receive the dead — estimated at 28, including two of the rescuers. That nightmarish situation was the basis for a four-hour simulation yesterday involving 1,500 police officers, firefighters and other emergency workers and tested the city’s ability to respond to a chemical emergency, though not necessarily a terrorist attack. Full Story