Drugs and Alcohol in the Rail Industry
Train Operating Companies are required to have a system for compliance with the Transport and Works act 1992 which makes it a criminal offence to work on the railway essentially above the drink drive limit. In response to this act the rail industry adopted the lower levels of alcohol (already quoted in this thread) to guarantee compliance with the act. Network Rail, LUL and th Train Operating Companies are compelled through their operator's licensing scheme (previously Railway Safety Cases) to have a system for random selection, pre-employment screening and 'for cause' screening.
In addition it is incumbent on contractors working on the infrastructure (through contract conditions and supplier accreditation) to have similar compatible system.
It is up to each contractror or operator to apply the rules which is subject to audit.