Preventing Opioid and Drug Impairment in Transportation Act This bill addresses drug and alcohol testing for transportation-related activities. Specifically, the bill requires the National Railroad Passenger Corporation (Amtrak) to report on methods it uses to ensure supervisors of employees in safety-sensitive positions receive the required training on how to detect drug and alcohol use and training on the rights and responsibilities of employees throughout the drug and alcohol testing process; the Department of Transportation (DOT) to determine whether to mandate that Amtrak locomotive engineers and conductors report arrests due to drug or alcohol offenses; DOT to report on the ability of pipeline companies that operate from Canada or Mexico into the United States to conduct the same drug and alcohol tests of safety-sensitive personnel that are required in the United States; DOT to amend its auditing program to improve the efficiency of certain drug and alcohol regulations related to testing contractors working in multiple states; the National Highway Traffic Safety Administration to study the ways it can reduce and better detect impaired driving, including marijuana- and opioid-impaired driving; DOT to conduct a study on the accuracy of on-site oral fluid screening for tetrahydrocannabinol and opiate presence in order to reduce the potential impact on traffic safety due to drug and polysubstance-impaired drivers; the Government Accountability Office to review DOT's process for setting guidelines and drug testing requirements for transportation employees; reports on whether to add fentanyl to the drug testing panel and the status of the guidelines for hair testing of transportation employees; DOT to report on the current status and operational potential of the Driver Alcohol Detection System for Safety Program's alcohol detection technology; and DOT to identify the barriers states face in submitting toxicology results of fatally injured drivers, provide recommendations to address such barriers, and establish minimum guidelines for toxicological investigations.