Network Rail have emergency response crews.

For an rail network that runs 24/7 they're going to have crews specifically to wake up should there be a problem on the busiest sections of mainline as this hoax indicated there were. That's a significant amount of dollars burned if they close the line due to a citizen reporting heavy damage to the bridge, and just waiting until 8am on the next business day to actually look at anything.

I strongly suspect what happened was they woke up their on-call inspectors (or scrambled an inspector who worked nights, which a rail network may very well have) informed them of photos circulating showing significant structural damage to this 150 year old viaduct, so they roll up and see the exact same viaduct in the exact same shape it's been in for their entire life and call up their boss and say "oy you wakin me up for this shiv? The bridge is bloody fine! Check your sauces mate!" (And after reporting that it was a hoax probably went and did a more thorough inspection to make sure their bases were covered)