Under the Aviation section of the 2003 Railways and Transport Safety Act, a police officer may arrest a person he believes to be under the influence of drink or drugs. Subsequently a specimen can be required to confirm whether or not this is the case.
I can only imagine that there is no access to a special graduated unit, so therefore the person would have to be arrested in order to be tested at the Police Station as the normal road traffic one would not indicate the level accurately ?
In the case of Railway staff, the BT Police will breathalyse on site using an approved unit which will indicate whether or not the person exceeds the legal limit or they may wait for the independent testing agent to undertake the test which gives an immediate result.
If they do then they are arrested and further specimens taken at the Police Station if the Police intend to take action. Not all cases of staff being under the influence will result in a Police Prosecution.