30 tons of explosive
ammonium nitrate has gone missing from the rail car in which is was being transported, somewhere between Cheyenne, Wyoming, and the Mojave Desert, en route to a destination in California.
Your mission, should you choose to accept it, is to discover who took it, why, and how. Ammonium nitrate was the explosive used in the 1995 Oklahoma City bombing, as well as the 1947 Texas City disaster, one of history's largest non-nuclear explosions. So terrorism is an obvious thought. The mundane use of the chemical as fertilizer was banned by Pakistan in an attempt to keep it out of the hands of militants there. It's also been used in rocket propellant, in case you need amateur skullduggery IN SPACE, and refrigeration and cold packs as its dissolution in water is highly endothermic, if cold is the best way to keep the Mojave Elder Thing in torpor. Possibly appealing locations along the likely route could include Colorado Springs, Los Alamos, and Las Vegas / Hoover Dam.