OMAHA, Neb. (AP) — The nation’s largest railroad union endorsed Union Pacific’s proposed $85 billion acquisition of Norfolk Southern Monday after securing promises to protect jobs. Other unions and chemical makers that rely on the railroads are still expressing concerns about the deal. The SMART-TD union that represents conductors and other rail workers said Union Pacific promised not to lay off any of its workers as a result of the merger and protect their jobs throughout their careers. But the head of one of the next biggest unions said he doesn’t think this deal does nearly enough to protect rail jobs, so he’s not ready to support the merger…