Let us ignore for the moment that “bridge” HHH 410 appears to consists of some red pillars sticking out over the river Thames and is unlikely to affect the safety of trains.
It is a bit difficult to imagine a scenario where the watercourse in question, the Thames, is blocked.
In the event of the Thames freezing solid or debris sufficient to prevent the river free flowing I am sure that the 'Rail Authority' would be aware of it without the need of a phone call from a passing member of the public.