49 CFR Part 40
49 CFR Part 40 is a collection of DOT regulations that dictate the conditions and rules regarding drug and alcohol testing for DOT-regulated entities. The part contains how tests should be conducted, who should be tested, what they should be tested for, and the various responsibilities and requirements for Service Agents in the testing process. The Code of Federal Regulations has parts providing further rules and guidelines for each specific subagency of the DOT including the FMCSA, FRA, FAA, FTA, PHMSA, and USCG. Regulations for each agency can be found on their respective pages.
49 CFR Part 40 can be read in full at the DOT website here.