Why is Positive Train Control such a train wreck?

Source: Metrolink Trains
This article in Governing Magazine is the best I have seen on Positive Train Control, and why its implementation has been such a failure.

How Soon Before Trains Have the Technology That Would Have Prevented Philadelphia’s Crash?

For more on this topic, see yesterday’s NPR story – Positive Train Control: The Tech That Could’ve Prevented Amtrak Derailment
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