Comment by teraflop

They do have such sensors, and have for many years! Japan's shinkansen pioneered this tech in the 1960s (and has an essentially perfect safety record). It's just that by necessity, the sensing is done through the tracks, rather than directly by the train cars.

You might be imagining something like the automated emergency braking that's used in automobiles, where radar is used to detect an imminent collision. But high-speed trains have stopping distances measured in miles, and it's not feasible for one train to detect another at such distances (especially e.g. on parallel tracks or around curves).

So instead the train positions have to be monitored and relayed by sensors in the tracks, which necessitates a coordination system. And it seems that that system is what broke down. The automated system should have been (and presumably was) fail-safe, but instead of stopping trains until it was fixed, the operators chose to override it and use a much more fallible manual signaling system.

> Stuff like this just further enhances my fear of collective travelling.

I'm sure you're already aware that this is an irrational fear. As bad as this accident is, and as reprehensible as it was that it was allowed to happen, I'm pretty sure there have still been significantly fewer fatalities per passenger-km in Spain this year than there have been on US highways. It's just that we've become numb to the fact that 100 Americans die in car accidents every day.