[Marxism] Some Info on Safe Train Separation

[S. Artesian]

For Les and David in particular:

And a note, "flickering" track circuits are not unheard of the railroad 
industry.  For the general railroad network, FRA requires periodic testing 
to confirm the integrity of track circuits, lack of grounds, absence 
conductivity of stray currents, or ground conductivity of current across 
insulated joints.

In third rail DC-electrified territory, AC track circuits are used and can 
be coded in a range of precise frequencies to give different speed 
authorizations based on conditions ahead. This is the basis for cab signal 
systems.  With this system, impedance bonds are installed to maintain the 
traction power continuity but isolate the track circuits to specific defined 
"blocks" of track.

Certainly an intermittent failure might go unnoticed if no train was 
following a train in the section of track that had the defective 
circuitry... but the fact that maintenance was performed on just that 
section of track 5 days before the collision-- and that an impedance bond 
was changed, would indicate to me that there was a known indication of a 
problem before the actual event.

When AC traction is utilized, or AC is generated by on-board equipment to 
provide "hotel power"-- lighting, heating, air-conditioning-- the frequency 
generated by the equipment must not be in the frequency range utilized by 
the track circuits to determine occupancy, otherwise a train can be standing 
on the track and the equipment on-board a following train could "overpower" 
the track circuitry indication and provide a "false clear" cab signal to the 
following train.
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