At the scene…

Few passengers even notice the short Wallers Ash Tunnel. Commuter trains are beneath the ground no longer than a handful of seconds, and the approach heading north is in a deep, green cutting.

It’s a busy railway bottleneck. Two tracks handle all the passenger and freight traffic heading to Eastleigh, Southampton, Bournemouth and beyond.

High above the tunnel portal, the only access is on foot. The morning after the landslip, with rain still pouring down, it was an unpleasant trudge through deep mud to get there.

I’m no geotechnical engineer, but listening to the workers on site the cause seemed clear.

Large quantities of surface runoff from neighbouring fields and the copse above the tunnel accumulated in a small hollow. In summer, this would normally be parched dry. But a month’s rain had fallen in an afternoon, and it had nowhere to go but down the steep side of the cutting, deeply covered in undergrowth that was rapidly washed away. It must have been quite a sight - and taken just a few minutes.

There’s no record of recent landslips here. Until now, the ground appeared stable.

Academics have long been warning that hotter summers and more intense periods of rainfall associated with a changing climate would bring more such incidents of debris wash to the railway.

They are harder to predict than more common landslides, in which a loose surface shifts over a more stable layer beneath.

Now we’ve had at least two in less than three weeks. Fortunately, the CrossCountry train did not derail when it struck the mud. It could easily have been much worse.