Prior to the formation of the Bureau of Explosives for the Safe Transportation of explosives and Other Dangerous Articles.in 1907 the were essentially no regulations for transportation of explosive by rail. The Bureau developed requirements for testing, packaging and loading of each type of explosive as well as train placement and switching requirements. These requirements were adopted by the ICC as regulations in the teens and the Bureau continued writing the regulations for the ICC until the Department of Transportation was formed in 1966.
The regulations for describing, marking, labeling, placarding, classification and packaging of explosives are found in Part 172 and 173 of Title 49 Code of Federal Regulations. These requirements have significantly changed since the 60's. Requirements for loading, train placement and switching are fount Part 174 of 49CFR with the exception of using the current classification system of Explosives 1.1, 1.2, 1.3, 1.4, 1.5, and 1.6 versus Class A, B or C explosives.
If there is interest, I can post the requirements from 19123 and 1940.