Please point to my post that suggests that.I'm beginning to wonder whether you're being deliberately obtuse.
The police aren't the ones making risk determinations. They do, however, have the job of communicating those recommendations to other agencies (including the railway) involved in the incident. In the specific case of the Hope Valley line, expert advice was communicated, by the police, to the railway. That's how the process is supposed to work.
You now appear to be suggesting that it's best to wait until there is visible damage to critical infrastructure (e.g. bridges) before closing that infrastructure. Rarely, if ever, have I encountered such blinding stupidity. By the time damage is visible, it's too late (if you need an example of that just look at the dam itself).
Continuing to run trains through the Hope Valley is a risk that simply doesn't need to be taken. That's the expert advice, and the railway is right to follow it. Whether troublemaking bystanders like yourself agree or not.