A non issue in my view.
Legionella is only dangerous in fine water sprays that can be inhaled.
Cooling towers, showers, and similar equipment are high risk.
Decorative water features and spray taps carry some risk.
Warm water is the risk, below 20 degrees or above 55 degrees the risks are minimal.
Water stored between 20 degrees and 55 degrees carries some risk, but only if sprayed in fine droplets and then inhaled. As might occur in a shower.
Water supplied at less than 20 degrees and then heated to say 40 degrees at the point of use without storage (as in an electric shower) carries almost no risk.
Water that is constantly re-used or recirculated as in a cooling tower or decorative water feature. is high risk.
I have never heard of anyone catching Legionnaires disease from toilet flushing water, or from hand washing water in the toilet on a train or indeed anywhere else.
I presume that all
RMT▸ members have their home toilet flush cisterns regularly tested for legionella, and disinfected as a precaution.
https://www.bbc.co.uk/news/uk-england-london-58298711