> A few sensors every few meters. The US has a total length of about 300 000km of railway network and that probably counts multi-track connections once. And it

> A few sensors every few meters. The US has a total length of about 300 000km of railway network and that probably counts multi-track connections once. And it doesn’t include subway or anything. So that would be a lot of sensors that need to be placed and maintained. And you’d need to except some failure of sensors, so you cannot just stop if a single sensor goes haywire. It’s certainly not impossible, but also not trivially done.