Madrid-Seville train collision: safety failures under investigation

How can one train catch up with another on a high- speed line?

To understand it, we need to look at the safety chain that should have prevented it.

The track is divided into sections several kilometers long, called blocks.

The rule is simple: in each block, only one train.

Electrical circuits in the rails detect whether the block is occupied. If it is, the train behind cannot enter.

On the Madrid- Seville line, this is monitored by a system called LZB. It communicates with the train constantly via a cable laid between the rails.

It knows which blocks are occupied, so it calculates how far each train can go and at what speed. The driver sees it on the screen.

And if there is a train ahead, the LZB sets a stopping point before reaching it.

If the driver does not brake in time, the system brakes the train itself, automatically.

But today, at 8:32 a.m., the LZB control center in the area has failed.

Six trains have come to a standstill over 90 kilometers.

When the LZB fails, plan B kicks in: trackside signals and a system called digital ASFA.

Beacons on the ground check that the train obeys every signal.

If it passes a red one without permission, it brakes it. And here is the key. A signal at red can be passed if the control center authorizes it.

It is called authorized passing. And in that mode, the ASFA only monitors speed.

It does not know whether there is a train ahead.

That is why the rules require on- sight operation.

Slowly, at around 40 kilometers per hour, so you can stop for anything you see.

The main hypothesis, according to the Transport Ministry, is that Atocha control center authorized it to proceed with the Iryo still stopped ahead.

The minister has called it an inexplicable error, but an order from Atocha is not enough for this to happen.

Even if control authorizes it to proceed, the backup safeguards are still there.

Proceeding on sight requires going slowly.

And ASFA, when passing a signal at danger with authorization, monitors that speed.

If the train exceeds it, it should brake it.

What is still unknown is why that order was given, what the driver saw, and why none of those safeguards stopped the train.

That is now being investigated by the Railway Accident Investigation Commission.

A system like this has several layers, so that a single failure is not enough. Today, for reasons that still need to be clarified, Several failed at once. Our thoughts are with the victims and their families.

Very soon, the full and more detailed version in a long-form video.