The July 1951 issue of Popular Science magazine described in detail
the cab signal and speed control systems available at the time.
One of the systems was the automatic speed control in use on
parts of the PRR and LIRR.  It was mandated on the LIRR after two
very deadly wrecks.  In that, an axle governor measures speed
and it is compared to the signal indication, if the speed is in
excess, the train is stopped.

I have no idea how these compares with the capabilities of modern
ACESES on the NEC, but that automatic speed control seems to cover
most situations.

I would think if it had been in place, it would've prevented the
recent Metro North and Amtrak NEC accidents were excessive speed
was the cause.

The article may be found at:
https://books.google.com/books?id=0SADAAAAMBAJ&lpg=PA52&dq=Penns
lvania%20railroad%20cab%20signal&pg=PA50#v=onepage&q=Pennsylvani
%20railroad%20cab%20signal&f=false

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