The water in steam railway towers is going to just get boiled, condensed and ejected into the atmosphere (thereby killing any such pathogens) and hence there is no such risk.
The problem is not with the steam engine emissions which as you say are effectively sterilised, but with the potential for a colony of legionella to grow inside the tank and pipes. In this case the water towers are generally outdoors and as such for much of the year are below the 25degC minimum at which colonies can grow.
Cooling towers are a major risk as many do circulate water in the 25-45deg "danger range" when rapid colony growth can occur and the towers release aerosol clouds into the local area which if infected can present a real danger to the public as well as staff. This is still believed to be the source of the Edinburgh outbreak, and it is notable that as soon as the suspect towers were sterlized the outbreak died down.
Beyond cooling systems any indoor storage of non-treated water (commonplace in commercial and industrial buildings) at room temperature or above (25-45degC) is a potential risk, albeit (assuming there is no mechanism to create an aerosol cloud) only to staff and occupants. There have been a number of cases traced back to infected shower or washing systems, and as a results such systems need to be tested annually. One should never drink water from taps in commercial and industrial premises without first confirming it is drinking water as ingestion does present a low risk.
Our domestic water tanks (and indeed any drinking water system) should be safe as the public water supply is treated with either cloramide or chlorine either of which kills the legionella bacteria.
For rail the engine coolant and steam is far too hot and the air conditioners use refrigerants rather than water. The only potential source would be the water supply for the toilets, but hopefully that is managed by treating or regular testing. Even if infected there is no mechanism for the bacterial to be released into the wider environment so the infection would be limited to passengers and staff.