If the sensor in X is broken, then the server will say "I cannot prove it is safe to move through X", and the train will stop.
I'm not a transportation engineer however, so I'll defer to anyone with actual experience in the field. But the idea is that our sensors are so effective today, that it is better to "prove" each leg of the journey is safe with "positive train control", rather than the opposite normal approach. (Ex: Sensor detects a problem, which stops the train).
That is to say: all sensors in the USA are in positive-train control mode. Trains will therefore stop if any sensor malfunctions. We're now in a "default stop" state for all trains, unless those sensors are working, deployed across USA.
The answer is simple: a 2nd or 3rd track for redundancy. Which is needed for maintenance purposes anyway. And flamethrowers for snowy areas.
From this perspective, the infrastructure to prove a line of track is safe is just "part of the cost" of a modern rail line. And any such failure is fixed as part of the maintenance costs.
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In any case, if you really wanted to stop a train during rush hour or something, I suggest a crowbar on the rail lines. There's easier ways to derail a train, and sabotage is a known problem. Physical security measures are available, camera systems etc. etc.
But a saboteur with a crowbar is roughly the same problem as too much snow on any given track. We definitely have more to worry from Mother Nature rather than our fellow man (at least.. for now).
