While federal investigators never determined the cause of the 1974 Apple Yard explosion, a 1975 report from the National Transportation Safety Board notes three possibilities:
- A railcar coupler separation process that penetrated the tank holding monomethylamine nitrate.
- Cavities created in the metal skin of the tank, followed by recompression within the monomethylamine nitrate.
- Compression of an air bubble encapsulated in the monomethylamine nitrate or trapped in the air space at the top of the tank.
Monomethylamine nitrate is…