Crews from Texas clambered amid the volatile wreckage of derailed propane tankers, painstakingly building an earthen pit to burn siphoned fuel that has raised fears of an explosion. The 1,700 residents of Weyauwega, Wis., evacuated Monday were told it might not be safe to return home for three weeks because the propane left in a blazing train wreck would have to burn off by itself. There was no word on prospects for ending the evacuation. Reconnaissance teams also examined the wreckage and found enough propane had burned off to allow workers near the scene. Texas crews were being used because of their expertise in fighting oil well fires.