a moving freight train hits a parked train at the texarkana depot, causing a derailment but not an explosionld you call back and see if they can ascertain exactly which car is overturned and maybe what the contents of the car was? >> it's not like a regular car accident. it can be thousands of gallons of very toxic chemicals up to maybe nothing leaking at all. kind of have to get there and investigate and determine what's going on. >> officer mcadams is one of the first on the scene. >> i've got some kind of chemical smell. i'm backing up. it's slowly drifting further south. >> he notices gas seeping out and spreading in a dangerous vapor cloud. >> i see kind of what looked like a light fog, chemical that was leaking was heavier than air so it kept to the low ground. the chemical cloud was actually following me, circling me up, so i retreated back. >> the fire department also responds, but battalion chief rick pavick doesn't see the vapor cloud. he's taken completely by surprise when the gas finds an ignition source and erupts 500 feet away. >> it was real crazy. it was amazing, is what it w