OMAHA, Neb. (AP) — Federal inspectors have twice found hundreds of defects in the locomotives and railcars Union Pacific uses at the world’s largest railyard in Nebraska, but none of those seem to explain why a shipping container filled with toxic acid exploded there this fall. Investigators haven’t confirmed the cause of the Sept. 14 blast in a remote corner of the railroad's Bailey Yard in North Platte, Nebraska, about 250 miles west of Omaha.