falling into a park below. nearly 100 firefighters raced to the scene, as flames raced to engulf cars loaded with lumber, grain and other products. >> the fire is traveling north along the train bridge, which is making it extraordinarily difficult for our firefighters to fight. >> reporter: union pacific says tanker cars that fell off the bridge contain a hazardous and highly flammable chemical. the fire department says one tanker is leaking and is using heavy equipment to pull the car upright, stop the leak and prevent another fire. as flames raged, a light rail passenger train on a nearby bridge slowly pushed its way through the smoke. valley metro rail telling us it was safer for this final train to pass through than stop on the bridge. federal investigators now working to figure out why the derailment happened. and david, firefighters tell us that the chemical leaking from that tanker ignites very easily, which is what makes this such a delicate cleanup operation. they say it could last for days. and keep in mind, the crews are doing all this work in the heat. temperatures hovering around 110 degrees. david?