Hulk was living on the streets of Los Angeles near some train tracks when he was very badly injured, most likely from being hit by a car or a train. He limped along the tracks, struggling to walk, until finally a group of people noticed him and immediately called for help.
Hope for Paws got the call about Hulk and quickly drove out to try and help him. The group of people who had called were still there when rescuers arrived, eager to help get the poor pit bull to safety. The tracks that Hulk was living next to were active, meaning trains could come by at any moment — so someone put a call in to the sheriff in hopes that he could stop the trains long enough for the rescuers to get Hulk.