Risk assessments consider the combination of the
probability of an event and the
impact of that event. In this case there is clearly a reasonable probability that the dam will survive, now the water level has passed its peak. But the impact of a catastrophic failure on Whaley Bridge itself would be devastating. The water level could rise too quickly for people to escape, hence the "risk to life" assessment that has resulted in precautionary evacuation.
Further downstream there would be more warning time, the water might rise more slowly to a lesser depth and/or flow less swiftly. This reduces the impact, and so the risk, although the probability is the same.
People trapped in a train could find it more difficult to escape the rising floodwaters than those in buildings or road vehicles - this might make the risk assessment for the railway higher than for the surrounding area.
Unrelated to this specific incident, the Enviroment Agency publishes long term reservoir flood risk maps for reservoirs all over the country. That for Whaley Bridge can be viewed at
https://flood-warning-information.s...g=401178.33&northing=381319.65&map=Reservoirs and can be zoomed and panned to view Furness Vale, New Mills and other places downstream. The "detailed view" enables the predicted depth and speed of the floodwater to be viewed, as well as the extent.
This map shows the worst case flood risk from any of the reservoirs in the area, including Combes and Fernilee, so may well be pessimistic in respect of Toddbrook. Nevertheless it indicates a potential for flooding right down the Goyt and Mersey valleys, through Marple, Stockport and Ashton upon Mersey, up to the discharge of the Mersey into the Ship Canal at Carrington. With these rivers already at a high level, it would not take much extra water for them to overtop their floodbanks.