The conductors, mechanics and engineers who operate Amtrak's trains have been testing positive for drugs and alcohol more and more frequently over the last six years, a government watchdog said Friday. And Amtrak's management isn't doing enough to stop it.

Drug and alcohol use by Amtrak operating employees in safety-sensitive positions far exceeds the national average for the railroad industry, Amtrak's inspector general said in a report warning of serious safety risks.

Amtrak's mechanics and signal operators had the highest rate in 2011, testing positive for drugs four times as often as those working for other railroads. Although Amtrak also tests for alcohol, the larger problem in recent years has been with drugs — specifically cocaine and marijuana