Ammonium nitrate is commonly used as fertilizer. It’s also an ingredient in high explosives and was used in the homemade bomb detonated in the 1995 attack on the Murrah Federal Building in Oklahoma City.
Dyno Nobel says it believes the material, transported in pellet form in a covered hopper car similar to those used to ship coal, fell from the car on the way to a rail siding (a short track connecting with the main track) called Saltdale about 30 miles from the town of Mojave in eastern Kern County.
“The railcar was sealed when it left the Cheyenne facility, and the seals were still intact when it arrived in Saltdale. The initial assessment is that a leak through the bottom gate on the railcar may have developed in transit,” the company said through a spokesperson.
A Federal Railroad Administration representative, though, says the investigation points to one of the hopper car gates not being properly closed.
Dyno Nobel says the trip lasted two weeks and included multiple stops. The company says it had “limited control” over the railcar as Union Pacific moved it through the country. It says the railcar is being transported back to Wyoming for inspection. And it says it hopes to understand how the shipment was lost and will work to prevent something similar happening again.
The Federal Railroad Administration, the California Public Utilities Commission, Union Pacific and Dyno Nobel are investigating the incident, according to their representatives.
Congress passed a law in 2007 to regulate the sale and transfer of ammonium nitrate to prevent its use in acts of terrorism. The Department of Homeland Security issued proposed regulations in 2011 (PDF) but stopped short of formally adopting them.

https://www.kqed.org/news/11949697/california-officials-trying-to-track-down-missing-30-ton-shipment-of-explosive-chemicals