GOVIA Thameslink Railway is working closely with the Office of Rail & Road and has established an action plan after a small number of toilets on Class 700 units tested positive for legionella.
GTR says the traces were detected during tests which took place as part of ongoing risk assessments for the fleet.
The process includes looking at appropriate controls and mitigation measures for bacteria such as legionella.
Steve White, GTR chief operating officer, said: “While it is extremely unlikely this would cause any harm to passengers or colleagues because of the low likelihood of transmission, our process is to straightaway lock the toilets out of use. The trains are subsequently taken out of service and the toilets completely drained, chlorinated and their tanks refilled.”